Duality Principles and Numerical Procedures for a Large Class of Non-convex Models in the Calculus of Variations

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Duality Principles and Numerical Procedures for a Large Class of Non-convex Models in the Calculus of Variations

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Fabio Botelho^*

This version is not peer-reviewed

Submitted:

28 April 2023

Posted:

29 April 2023

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Abstract

This article develops duality principles and numerical results for a large class of non-convex variational models. The main results are based on fundamental tools of convex analysis, duality theory and calculus of variations. More specifically the approach is established for a class of non-convex functionals similar as those found in some models in phase transition. Finally, in the last section we present a concerning numerical example and the respective software.

Keywords:

Subject: Computer Science and Mathematics - Applied Mathematics

MSC: 49N15

1. Introduction

In this section we establish a dual formulation for a large class of models in non-convex optimization.

The main duality principle is applied to double well models similar as those found in the phase transition theory.

Such results are based on the works of J.J. Telega and W.R. Bielski [2,3,14,15] and on a D.C. optimization approach developed in Toland [16].

About the other references, details on the Sobolev spaces involved are found in [1]. Related results on convex analysis and duality theory are addressed in [5,6,7,9,13].

Finally, in this text we adopt the standard Einstein convention of summing up repeated indices, unless otherwise indicated.

In order to clarify the notation, here we introduce the definition of topological dual space.

Definition 1.1

(Topological dual spaces). Let U be a Banach space. We shall define its dual topological space, as the set of all linear continuous functionals defined on U. We suppose such a dual space of U, may be represented by another Banach space

U^{*}

, through a bilinear form

{〈 \cdot, \cdot 〉}_{U} : U \times U^{*} \to R

(here we are referring to standard representations of dual spaces of Sobolev and Lebesgue spaces). Thus, given

f : U \to R

linear and continuous, we assume the existence of a unique

u^{*} \in U^{*}

such that

\begin{matrix} f (u) = {〈 u, u^{*} 〉}_{U}, \forall u \in U . \end{matrix}

(1)

The norm of f , denoted by

{∥ f ∥}_{U^{*}}

, is defined as

\begin{matrix} {∥ f ∥}_{U^{*}} = sup_{u \in U} {| {〈 u, u^{*} 〉}_{U} {| : ∥ u ∥}_{U} \leq 1} \equiv ∥ u^{*} ∥_{U^{*}} . \end{matrix}

(2)

At this point we start to describe the primal and dual variational formulations.

2. A general duality principle non-convex optimization

In this section we present a duality principle applicable to a model in phase transition.

This case corresponds to the vectorial one in the calculus of variations.

Let

Ω \subset R^{n}

be an open, bounded, connected set with a regular (Lipschitzian) boundary denoted by

\partial Ω .

Consider a functional

J : V \to R

where

J (u) = F (\nabla u_{1}, \dots, \nabla u_{N}) + G (u_{1}, \dots, u_{N}) - {〈 u_{i}, f_{i} 〉}_{L^{2}},

and where

V = {u = (u_{1}, \dots, u_{N}) \in W^{1, p} (Ω; R^{N}) : u = u_{0} on \partial Ω},

f \in L^{2} (Ω; R^{N}),

and

1 < p < + \infty .

We assume there exists

α \in R

such that

α = inf_{u \in V} J (u) .

Moreover, suppose F and G are Fréchet differentiable but not necessarily convex. A global optimum point may not be attained for J so that the problem of finding a global minimum for J may not be a solution.

Anyway, one question remains, how the minimizing sequences behave close the infimum of J.

We intend to use duality theory to approximately solve such a global optimization problem.

Denoting

V_{0} = W_{0}^{1, p} (Ω; R^{N})

Y_{1} = Y_{1}^{*} = L^{2} (Ω; R^{N \times n})

Y_{2} = Y_{2}^{*} = L^{2} (Ω; R^{N \times n})

Y_{3} = Y_{3}^{*} = L^{2} (Ω; R^{N}),

at this point we define,

F_{1} : V \times V_{0} \to R

G_{1} : V \to R

G_{2} : V \to R

G_{3} : V_{0} \to R

and

G_{4} : V \to R,

\begin{matrix} F_{1} (\nabla u, \nabla ϕ) & = & F (\nabla u_{1} + \nabla ϕ_{1}, \dots, \nabla u_{N} + \nabla ϕ_{N}) + \frac{K}{2} \int_{Ω} \nabla u_{j} \cdot \nabla u_{j} d x \\ + \frac{K_{2}}{2} \int_{Ω} \nabla ϕ_{j} \cdot \nabla ϕ_{j} d x \end{matrix}

(3)

and

G_{1} (u_{1}, \dots, u_{n}) = G (u_{1}, \dots, u_{N}) + \frac{K_{1}}{2} \int_{Ω} u_{j} u_{j} d x - {〈 u_{i}, f_{i} 〉}_{L^{2}},

G_{2} (\nabla u_{1}, \dots, \nabla u_{N}) = \frac{K_{1}}{2} \int_{Ω} \nabla u_{j} \cdot \nabla u_{j} d x,

G_{3} (\nabla ϕ_{1}, \dots, \nabla ϕ_{N}) = \frac{K_{2}}{2} \int_{Ω} \nabla ϕ_{j} \cdot \nabla ϕ_{j} d x,

and

G_{4} (u_{1}, \dots, u_{N}) = \frac{K_{1}}{2} \int_{Ω} u_{j} u_{j} d x .

Define now

J_{1} : V \times V_{0} \to R

J_{1} (u, ϕ) = F (\nabla u + \nabla ϕ) + G (u) - {〈 u_{i}, f_{i} 〉}_{L^{2}} .

Observe that

\begin{matrix} J_{1} (u, ϕ) & = & F_{1} (\nabla u, \nabla ϕ) + G_{1} (u) - G_{2} (\nabla u) - G_{3} (\nabla ϕ) - G_{4} (u) \\ \leq & F_{1} (\nabla u, \nabla ϕ) + G_{1} (u) - {〈 \nabla u, z_{1}^{*} 〉}_{L^{2}} - {〈 \nabla ϕ, z_{2}^{*} 〉}_{L^{2}} - {〈 u, z_{3}^{*} 〉}_{L^{2}} \\ + sup_{v_{1} \in Y_{1}} {{〈 v_{1}, z_{1}^{*} 〉}_{L^{2}} - G_{2} (v_{1})} \\ + sup_{v_{2} \in Y_{2}} {{〈 v_{2}, z_{2}^{*} 〉}_{L^{2}} - G_{3} (v_{2})} \\ + sup_{u \in V} {{〈 u, z_{3}^{*} 〉}_{L^{2}} - G_{4} (u)} \\ = & F_{1} (\nabla u, \nabla ϕ) + G_{1} (u) - {〈 \nabla u, z_{1}^{*} 〉}_{L^{2}} - {〈 \nabla ϕ, z_{2}^{*} 〉}_{L^{2}} - {〈 u, z_{3}^{*} 〉}_{L^{2}} \\ + G_{2}^{*} (z_{1}^{*}) + G_{3}^{*} (z_{2}^{*}) + G_{4}^{*} (z_{3}^{*}) \\ = & J_{1}^{*} (u, ϕ, z^{*}), \end{matrix}

(4)

\forall u \in V, ϕ \in V_{0}, z^{*} = (z_{1}^{*}, z_{2}^{*}, z_{3}^{*}) \in Y^{*} = Y_{1}^{*} \times Y_{2}^{*} \times Y_{3}^{*}

Here we assume

K, K_{1}, K_{2}

are large enough so that

F_{1}

and

G_{1}

are convex.

Hence, from the general results in [16], we may infer that

\begin{matrix} inf_{(u, ϕ) \in V \times V_{0}} J (u, ϕ) & = & inf_{(u, ϕ, z^{*}) \in V \times V_{0} \times Y^{*}} J_{1}^{*} (u, ϕ, z^{*}) . \end{matrix}

(5)

On the other hand

inf_{u \in V} J (u) \geq inf_{(u, ϕ) \in V \times V_{0}} J_{1} (u, ϕ) \geq inf_{u \in V} Q_{J} (u) = inf_{u \in V} J (u),

where

Q_{J} (u)

refers to a standard quasi-convex regularization of J.

From these last two results we may obtain

inf_{u \in V} J (u) = inf_{(u, ϕ, z^{*}) \in V \times V_{0} \times Y^{*}} J_{1}^{*} (u, ϕ, z^{*}) .

Moreover, from standards results on convex analysis, we may have

\begin{matrix} inf_{u \in V} J_{1}^{*} (u, ϕ, z^{*}) & = & inf_{u \in V} {F_{1} (\nabla u, \nabla ϕ) + G_{1} (u) \\ - {〈 \nabla u, z_{1}^{*} 〉}_{L^{2}} - {〈 \nabla ϕ, z_{2}^{*} 〉}_{L^{2}} - {〈 u, z_{3}^{*} 〉}_{L^{2}} \\ + G_{2}^{*} (z_{1}^{*}) + G_{3}^{*} (z_{2}^{*}) + G_{4}^{*} (z_{3}^{*})} \\ = & sup_{(v_{1}^{*}, v_{2}^{*}) \in C^{*}} {- F_{1}^{*} (v_{1}^{*} + z_{1}^{*}, \nabla ϕ) - G_{1}^{*} (v_{2}^{*} + z_{3}^{*}) - {〈 \nabla ϕ, z_{2}^{*} 〉}_{L^{2}} \\ + G_{2}^{*} (z_{1}^{*}) + G_{3}^{*} (z_{2}^{*}) + G_{4}^{*} (z_{3}^{*})}, \end{matrix}

(6)

where

C^{*} = {v^{*} = (v_{1}^{*}, v_{2}^{*}) \in Y_{1}^{*} \times Y_{3}^{*} : - div {(v_{1}^{*})}_{i} + {(v_{2}^{*})}_{i} = 0, \forall i \in {1, \dots, N}},

F_{1}^{*} (v_{1}^{*} + z_{1}^{*}, \nabla ϕ) = sup_{v_{1} \in Y_{1}} {{〈 v_{1}, z_{1}^{*} + v_{1}^{*} 〉}_{L^{2}} - F_{1} (v_{1}, \nabla ϕ)},

and

G_{1}^{*} (v_{2}^{*} + z_{2}^{*}) = sup_{u \in V} {{〈 u, v_{2}^{*} + z_{2}^{*} 〉}_{L^{2}} - G_{1} (u)} .

Thus, defining

J_{2}^{*} (ϕ, z^{*}, v^{*}) = F_{1}^{*} (v_{1}^{*} + z_{1}^{*}, \nabla ϕ) - G_{1}^{*} (v_{2}^{*} + z_{3}^{*}) - {〈 \nabla ϕ, z_{2}^{*} 〉}_{L^{2}} + G_{2}^{*} (z_{1}^{*}) + G_{3}^{*} (z_{2}^{*}) + G_{4}^{*} (z_{3}^{*}),

we have got

\begin{matrix} inf_{u \in V} J (u) & = & inf_{(u, ϕ) \in V \times V_{0}} J_{1} (u, ϕ) \\ = & inf_{(u, ϕ, z^{*}) \in V \times V_{0} \times Y^{*}} J_{1}^{*} (u, ϕ, z^{*}) \\ = & inf_{z^{*} \in Y^{*}} \{inf_{ϕ \in V_{0}} \{sup_{v^{*} \in C^{*}} J_{2}^{*} (ϕ, z^{*}, v^{*})\}\} . \end{matrix}

(7)

Finally, observe that

\begin{matrix} inf_{u \in V} J (u) \\ = & inf_{z^{*} \in Y^{*}} \{inf_{ϕ \in V_{0}} \{sup_{v^{*} \in C^{*}} J_{2}^{*} (ϕ, z^{*}, v^{*})\}\} \\ \geq & sup_{v^{*} \in C^{*}} \{inf_{(z^{*}, ϕ) \in Y^{*} \times V_{0}} J_{2}^{*} (ϕ, z^{*}, v^{*})\} . \end{matrix}

(8)

This last variational formulation corresponds to a concave relaxed formulation in

v^{*}

concerning the original primal formulation.

3. Another duality principle for a simpler related model in phase transition with a respective numerical example

In this section we present another duality principle for a related model in phase transition.

Let

Ω = [0, 1] \subset R

and consider a functional

J : V \to R

where

J (u) = \frac{1}{2} \int_{Ω} {({(u^{'})}^{2} - 1)}^{2} d x + \frac{1}{2} \int_{Ω} u^{2} d x - {〈 u, f 〉}_{L^{2}},

and where

V = {u \in W^{1, 4} (Ω) : u (0) = 0 and u (1) = 1 / 2}

and

f \in L^{2} (Ω) .

A global optimum point is not attained for J so that the problem of finding a global minimum for J has no solution.

Anyway, one question remains, how the minimizing sequences behave close the infimum of J.

We intend to use duality theory to approximately solve such a global optimization problem.

Denoting

V_{0} = W_{0}^{1, 4} (Ω)

, at this point we define,

F : V \to R

and

F_{1} : V \times V_{0} \to R

F (u) = \frac{1}{2} \int_{Ω} {({(u^{'})}^{2} - 1)}^{2} d x,

and

F_{1} (u, ϕ) = \frac{1}{2} \int_{Ω} {({(u^{'} + ϕ^{'})}^{2} - 1)}^{2} d x .

Observe

F (u) \geq inf_{ϕ \in V_{0}} F_{1} (u, ϕ) \geq Q_{F} (u), \forall u \in V,

where

Q_{F} (u)

refers to a quasi-convex regularization of

F .

We define also

F_{2} : V \times V_{0} \to R,

F_{3} : V \times V_{0} \to R

and

G : V \times V_{0} \to R

F_{2} (u, ϕ) = \frac{1}{2} \int_{Ω} {({(u^{'} + ϕ^{'})}^{2} - 1)}^{2} d x + \frac{1}{2} \int_{Ω} u^{2} d x - {〈 u, f 〉}_{L^{2}},

\begin{matrix} F_{3} (u, ϕ) & = & F_{2} (u, ϕ) + \frac{K}{2} \int_{Ω} {(u^{'})}^{2} d x \\ + \frac{K_{1}}{2} \int_{Ω} {(ϕ^{'})}^{2} d x \end{matrix}

(9)

and

\begin{matrix} G (u, ϕ) & = & \frac{K}{2} \int_{Ω} {(u^{'})}^{2} d x \\ + \frac{K_{1}}{2} \int_{Ω} {(ϕ^{'})}^{2} d x \end{matrix}

(10)

Observe that if

K > 0, K_{1} > 0

is large enough, both

F_{3}

and G are convex.

Denoting

Y = Y^{*} = L^{2} (Ω)

we also define the polar functional

G^{*} : Y^{*} \times Y^{*} \to R

G^{*} (v^{*}, v_{0}^{*}) = sup_{(u, ϕ) \in V \times V_{0}} {{〈 u, v^{*} 〉}_{L^{2}} + {〈 ϕ, v_{0}^{*} 〉}_{L^{2}} - G (u, ϕ)} .

Observe that

inf_{u \in U} J (u) \geq inf_{((u, ϕ), (v^{*}, v_{0}^{*})) \in V \times V_{0} \times {[Y^{*}]}^{2}} {G^{*} (v^{*}, v_{0}^{*}) - {〈 u, v^{*} 〉}_{L^{2}} - {〈 ϕ, v_{0}^{*} 〉}_{L^{2}} + F_{3} (u, ϕ)} .

With such results in mind, we define a relaxed primal dual variational formulation for the primal problem, represented by

J_{1}^{*} : V \times V_{0} \times {[Y^{*}]}^{2} \to R

, where

J_{1}^{*} (u, ϕ, v^{*}, v_{0}^{*}) = G^{*} (v^{*}, v_{0}^{*}) - {〈 u, v^{*} 〉}_{L^{2}} - {〈 ϕ, v_{0}^{*} 〉}_{L^{2}} + F_{3} (u, ϕ) .

Having defined such a functional, we may obtain numerical results by solving a sequence of convex auxiliary sub-problems, through the following algorithm.

Set $K \approx 150$ and $K_{1} = K / 20$ and $0 < ε ≪ 1 .$
Choose $(u_{1}, ϕ_{1}) \in V \times V_{0},$ such that $∥ u_{1} ∥_{1, \infty} ≪ K / 4$ and $∥ ϕ_{1} ∥_{1, \infty} ≪ K / 4 .$
Set $n = 1 .$
Calculate $(v_{n}^{*}, {(v_{0}^{*})}_{n})$ solution of the system of equations:

$\frac{\partial J_{1}^{*} (u_{n}, ϕ_{n}, v_{n}^{*}, {(v_{0}^{*})}_{n})}{\partial v^{*}} = 0$

and

$\frac{\partial J_{1}^{*} (u_{n}, ϕ_{n}, v_{n}^{*}, {(v_{0}^{*})}_{n})}{\partial v_{0}^{*}} = 0,$

that is

$\frac{\partial G^{*} (v_{n}^{*}, {(v_{0}^{*})}_{n})}{\partial v^{*}} - u_{n} = 0$

and

$\frac{\partial G^{*} (v_{n}^{*}, {(v_{0}^{*})}_{n})}{\partial v_{0}^{*}} - ϕ_{n} = 0$

so that

$v_{n}^{*} = \frac{\partial G (u_{n}, ϕ_{n})}{\partial u}$

and

${(v_{0}^{*})}_{n}^{*} = \frac{\partial G (u_{n}, ϕ_{n})}{\partial ϕ}$
Calculate $(u_{n + 1}, ϕ_{n + 1})$ by solving the system of equations:

$\frac{\partial J_{1}^{*} (u_{n + 1}, ϕ_{n + 1}, v_{n}^{*}, {(v_{0}^{*})}_{n})}{\partial u} = 0$

and

$\frac{\partial J_{1}^{*} (u_{n + 1}, ϕ_{n + 1}, v_{n}^{*}, {(v_{0}^{*})}_{n})}{\partial ϕ} = 0$

that is

$- v_{n}^{*} + \frac{\partial F_{3} (u_{n + 1}, ϕ_{n + 1})}{\partial u} = 0$

and

$- {(v_{0}^{*})}_{n} + \frac{\partial F_{3} (u_{n + 1}, ϕ_{n + 1})}{\partial ϕ} = 0$
If $max {∥ u_{n} - u_{n + 1} ∥_{\infty}, ∥ ϕ_{n + 1} - ϕ_{n} ∥_{\infty}} \leq ε$ , then stop, else set $n : = n + 1$ and go to item 4.

For the case in which

f (x) = 0

, we have obtained numerical results for

K = 1500

and

K_{1} = K / 20

. For such a concerning solution

u_{0}

obtained, please see Figure 1. For the case in which

f (x) = sin (π x) / 2

, we have obtained numerical results for

K = 100

and

K_{1} = K / 20

. For such a concerning solution

u_{0}

obtained, please see Figure 2.

Remark 3.1.

Observe that the solutions obtained are approximate critical points. They are not, in a classical sense, the global solutions for the related optimization problems. Indeed, such solutions reflect the average behavior of weak cluster points for concerning minimizing sequences.

4. A convex dual variational formulation for a third similar model

In this section we present another duality principle for a third related model in phase transition.

Let

Ω = [0, 1] \subset R

and consider a functional

J : V \to R

where

J (u) = \frac{1}{2} \int_{Ω} min {{(u^{'} - 1)}^{2}, {(u^{'} + 1)}^{2}} d x + \frac{1}{2} \int_{Ω} u^{2} d x - {〈 u, f 〉}_{L^{2}},

and where

V = {u \in W^{1, 2} (Ω) : u (0) = 0 and u (1) = 1 / 2}

and

f \in L^{2} (Ω) .

A global optimum point is not attained for J so that the problem of finding a global minimum for J has no solution.

Anyway, one question remains, how the minimizing sequences behave close to the infimum of J.

We intend to use the duality theory to solve such a global optimization problem in an appropriate sense to be specified.

At this point we define,

F : V \to R

and

G : V \to R

\begin{matrix} F (u) & = & \frac{1}{2} \int_{Ω} min {{(u^{'} - 1)}^{2}, {(u^{'} + 1)}^{2}} d x \\ = & \frac{1}{2} \int_{Ω} {(u^{'})}^{2} d x - \int_{Ω} | u^{'} | d x + 1 / 2 \\ \equiv & F_{1} (u^{'}), \end{matrix}

(11)

and

G (u) = \frac{1}{2} \int_{Ω} u^{2} d x - {〈 u, f 〉}_{L^{2}} .

Denoting

Y = Y^{*} = L^{2} (Ω)

we also define the polar functional

F_{1}^{*} : Y^{*} \to R

and

G^{*} : Y^{*} \to R

\begin{matrix} F_{1}^{*} (v^{*}) & = & sup_{v \in Y} {{〈 v, v^{*} 〉}_{L^{2}} - F_{1} (v)} \\ = & \frac{1}{2} \int_{Ω} {(v^{*})}^{2} d x + \int_{Ω} | v^{*} | d x, \end{matrix}

(12)

and

\begin{matrix} G^{*} ({(v^{*})}^{'}) & = & sup_{u \in V} {- {〈 u^{'}, v^{*} 〉}_{L^{2}} - G (u)} \\ = & \frac{1}{2} \int_{Ω} {({(v^{*})}^{'} + f)}^{2} d x - \frac{1}{2} v^{*} (1) . \end{matrix}

(13)

Observe this is the scalar case of the calculus of variations, so that from the standard results on convex analysis, we have

inf_{u \in V} J (u) = max_{v^{*} \in Y^{*}} {- F_{1}^{*} (v^{*}) - G^{*} (- {(v^{*})}^{'})} .

Indeed, from the direct method of the calculus of variations, the maximum for the dual formulation is attained at some

{\hat{v}}^{*} \in Y^{*}

Moreover, the corresponding solution

u_{0} \in V

is obtained from the equation

u_{0} = \frac{\partial G ({({\hat{v}}^{*})}^{'})}{\partial {(v^{*})}^{'}} = {({\hat{v}}^{*})}^{'} + f .

Finally, the Euler-Lagrange equations for the dual problem stands for

\{\begin{matrix} {(v^{*})}^{″} + f^{'} - v^{*} - sign (v^{*}) = 0, & in Ω, \\ {(v^{*})}^{'} (0) = 0, {(v^{*})}^{'} (1) = 1 / 2, \end{matrix}

(14)

where

sign (v^{*} (x)) = 1

v^{*} (x) > 0,

sign (v^{*} (x)) = - 1,

v^{*} (x) < 0

and

- 1 \leq sign (v^{*} (x)) \leq 1,

v^{*} (x) = 0 .

We have computed the solutions

v^{*}

and corresponding solutions

u_{0} \in V

for the cases in which

f (x) = 0

and

f (x) = sin (π x) / 2 .

For the solution

u_{0} (x)

for the case in which

f (x) = 0

, please see Figure 3.

For the solution

u_{0} (x)

for the case in which

f (x) = sin (π x) / 2

, please see Figure 4.

Remark 4.1

Observe that such solutions

u_{0}

obtained are not the global solutions for the related primal optimization problems. Indeed, such solutions reflect the average behavior of weak cluster points for concerning minimizing sequences.

4.1. The algorithm through which we have obtained the numerical results

In this subsection we present the software in MATLAB through which we have obtained the last numerical results.

This algorithm is for solving the concerning Euler-Lagrange equations for the dual problem, that is, for solving the equation

\{\begin{matrix} {(v^{*})}^{″} + f^{'} - v^{*} - sign (v^{*}) = 0, & in Ω, \\ {(v^{*})}^{'} (0) = 0, {(v^{*})}^{'} (1) = 1 / 2 . \end{matrix}

(15)

Here the concerning software in MATLAB. We emphasize to have used the smooth approximation

| v^{*} | \approx \sqrt{{(v^{*})}^{2} + e_{1}},

where a small value for

e_{1}

is specified in the next lines.

*************************************

clear all
$m_{8} = 800;$ (number of nodes)
$d = 1 / m_{8};$
$e_{1} = 0.00001;$
$f o r i = 1 : m_{8}$

$y o (i, 1) = 0.01;$

$y_{1} (i, 1) = sin (π * i / m_{8}) / 2;$

$e n d;$
$f o r i = 1 : m_{8} - 1$

$d y_{1} (i, 1) = (y_{1} (i + 1, 1) - y_{1} (i, 1)) / d;$

$e n d;$
$f o r k = 1 : 3000$ (we have fixed the number of iterations)

$i = 1;$

$h_{3} = 1 / \sqrt{v o {(i, 1)}^{2} + e_{1}};$

$m_{12} = 1 + d^{2} * h_{3} + d^{2};$

$m_{50} (i) = 1 / m_{12};$

$z (i) = m_{50} (i) * (d y_{1} (i, 1) * d^{2});$
$f o r i = 2 : m_{8} - 1$

$h_{3} = 1 / \sqrt{v o {(i, 1)}^{2} + e_{1}};$

$m_{12} = 2 + h_{3} * d^{2} + d^{2} - m 50 (i - 1);$

$m 50 (i) = 1 / m_{12};$

$z (i) = m_{50} (i) * (z (i - 1) + d y_{1} (i, 1) * d^{2});$

$e n d;$
$v (m_{8}, 1) = (d / 2 + z (m_{8} - 1)) / (1 - m_{50} (m_{8} - 1));$
$f o r i = 1 : m_{8} - 1$

$v (m_{8} - i, 1) = m_{50} (m_{8} - i) * v (m_{8} - i + 1) + z (m_{8} - i);$

$e n d;$
$v (m_{8} / 2, 1)$
$v o = v;$

$e n d;$
$f o r i = 1 : m_{8} - 1$

$u (i, 1) = (v (i + 1, 1) - v (i, 1)) / d + y_{1} (i, 1);$

$e n d;$
$f o r i = 1 : m_{8} - 1$

$x (i) = i * d;$

$e n d;$

$p l o t (x, u (:, 1))$

********************************

5. An improvement of the convexity conditions for a non-convex related model through an approximate primal formulation

In this section we develop an approximate primal dual formulation suitable for a large class of variational models.

Here, the applications are for the Kirchhoff-Love plate model, which may be found in Ciarlet, [10].

At this point we start to describe the primal variational formulation.

Let

Ω \subset R^{2}

be an open, bounded, connected set which represents the middle surface of a plate of thickness h. The boundary of

Ω

, which is assumed to be regular (Lipschitzian), is denoted by

\partial Ω

. The vectorial basis related to the cartesian system

{x_{1}, x_{2}, x_{3}}

is denoted by

(a_{α}, a_{3})

, where

α = 1, 2

(in general Greek indices stand for 1 or 2), and where

a_{3}

is the vector normal to

Ω

, whereas

a_{1}

and

a_{2}

are orthogonal vectors parallel to

Ω .

Also,

n

is the outward normal to the plate surface.

The displacements will be denoted by

\hat{u} = {{\hat{u}}_{α}, {\hat{u}}_{3}} = {\hat{u}}_{α} a_{α} + {\hat{u}}_{3} a_{3} .

The Kirchhoff-Love relations are

\begin{matrix} {\hat{u}}_{α} (x_{1}, x_{2}, x_{3}) = u_{α} (x_{1}, x_{2}) - x_{3} w {(x_{1}, x_{2})}_{, α} \\ and {\hat{u}}_{3} (x_{1}, x_{2}, x_{3}) = w (x_{1}, x_{2}) . \end{matrix}

(16)

Here

- h / 2 \leq x_{3} \leq h / 2

so that we have

u = (u_{α}, w) \in U

where

\begin{matrix} U & = & \{u = (u_{α}, w) \in W^{1, 2} (Ω; R^{2}) \times W^{2, 2} (Ω), \\ u_{α} = w = \frac{\partial w}{\partial n} = 0 on \partial Ω\} \\ = & W_{0}^{1, 2} (Ω; R^{2}) \times W_{0}^{2, 2} (Ω) . \end{matrix}

It is worth emphasizing that the boundary conditions here specified refer to a clamped plate.

We also define the operator

Λ : U \to Y \times Y

, where

Y = Y^{*} = L^{2} (Ω; R^{2 \times 2})

, by

Λ (u) = {γ (u), κ (u)},

γ_{α β} (u) = \frac{u_{α, β} + u_{β, α}}{2} + \frac{w_{, α} w_{, β}}{2},

κ_{α β} (u) = - w_{, α β} .

The constitutive relations are given by

N_{α β} (u) = H_{α β λ μ} γ_{λ μ} (u),

(17)

M_{α β} (u) = h_{α β λ μ} κ_{λ μ} (u),

(18)

where:

{H_{α β λ μ}}

and

\{h_{α β λ μ} = \frac{h^{2}}{12} H_{α β λ μ}\}

, are symmetric positive definite fourth order tensors. From now on, we denote

{{\bar{H}}_{α β λ μ}} = {H_{α β λ μ}}^{- 1}

and

{{\bar{h}}_{α β λ μ}} = {h_{α β λ μ}}^{- 1}

Furthermore

{N_{α β}}

denote the membrane force tensor and

{M_{α β}}

the moment one. The plate stored energy, represented by

(G \circ Λ) : U \to R

is expressed by

(G \circ Λ) (u) = \frac{1}{2} \int_{Ω} N_{α β} (u) γ_{α β} (u) d x + \frac{1}{2} \int_{Ω} M_{α β} (u) κ_{α β} (u) d x

(19)

and the external work, represented by

F : U \to R

, is given by

F (u) = {〈 w, P 〉}_{L^{2}} + {〈 u_{α}, P_{α} 〉}_{L^{2}},

(20)

where

P, P_{1}, P_{2} \in L^{2} (Ω)

are external loads in the directions

a_{3}

a_{1}

and

a_{2}

respectively. The potential energy, denoted by

J : U \to R

is expressed by:

J (u) = (G \circ Λ) (u) - F (u)

Define now

J_{3} : \tilde{U} \to R

J_{3} (u) = J (u) + J_{5} (w) .

where

J_{5} (w) = 10 \int_{Ω} \frac{a^{K b w}}{ln (a) K^{3 / 2}} d x + 10 \int_{Ω} \frac{a^{- K (b w - 1 / 100)}}{ln (a) K^{3 / 2}} d x .

In such a case for

a = 2.71

K = 185

b = P / | P |

Ω

and

\tilde{U} = {{u \in U : ∥ w ∥}_{\infty} \leq 0.01 and P w \geq 0 a . e . in Ω},

we get

\begin{matrix} \frac{\partial J_{3} (u)}{\partial w} & = & \frac{\partial J (u)}{\partial w} + \frac{\partial J_{5} (u)}{\partial w} \\ \approx & \frac{\partial J (u)}{\partial w} + O (\pm 3.0), \end{matrix}

(21)

and

\begin{matrix} \frac{\partial^{2} J_{3} (u)}{\partial w^{2}} & = & \frac{\partial^{2} J (u)}{\partial w^{2}} + \frac{\partial^{2} J_{5} (u)}{\partial w^{2}} \\ \approx & \frac{\partial^{2} J (u)}{\partial w^{2}} + O (850) . \end{matrix}

(22)

This new functional

J_{3}

has a relevant improvement in the convexity conditions concerning the previous functional J.

Indeed, we have obtained a gain in positiveness for the second variation

\frac{\partial^{2} J (u)}{\partial w^{2}},

which has increased of order

O (700 - 1000) .

Moreover the difference between the approximate and exact equation

\frac{\partial J (u)}{\partial w} = 0

is of order

O (\pm 3.0)

which corresponds to a small perturbation in the original equation for a load of

P = 1500 N / m^{2},

for example. Summarizing, the exact equation may be approximately solved in an appropriate sense.

6. An approximate convex variational formulation for another related model

In this section, we obtain an approximate convex variational formulation for a related model, more specifically, for a Ginzburg-Landau type equation.

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular (Lipschitzian) boundary denoted by

\partial Ω .

Consider a functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(23)

where

γ > 0

α > 0

β > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

We define

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq 1},

and

V_{1} = V_{2} \cap A^{+} .

At this point we define

v = u / 10

so that

\begin{matrix} J (u) & = & J_{1} (v) \\ = & \frac{10^{2} γ}{2} \int_{Ω} \nabla v \cdot \nabla v d x + \frac{α}{2} \int_{Ω} ({(10 v)}^{2} - β) d x \\ - {〈 10 v, f 〉}_{L^{2}} . \end{matrix}

(24)

Moreover we define

\begin{matrix} J_{2} (v) & = & \frac{1}{10} J_{1} (v) \\ = & \frac{10 γ}{2} \int_{Ω} \nabla v \cdot \nabla v d x + \frac{α}{20} \int_{Ω} ({(10 v)}^{2} - β) d x \\ - {〈 v, f 〉}_{L^{2}}, \end{matrix}

(25)

and

J_{3} : U_{3} \to R

where

J_{3} (v) = J_{2} (v) + J_{5} (v)

where

J_{5} (v) = K_{1} (\int_{Ω} \frac{a^{K^{3} (5 b w)}}{ln (a) K^{4}} d x + \int_{Ω} \frac{a^{- K^{3} (5 b w - 0.5)}}{ln (a) K^{4}} d x) .

Here

K_{1} = 1 / 360

a = 2.71

K = 2

b = f / | f |

Ω

and

U_{2} = {v \in V : f v \geq 0, a . e . in Ω},

U_{4} = {{v \in V : ∥ v ∥}_{\infty} \leq 1 / 10},

and

U_{3} = U_{2} \cap U_{4} .

Thus, with such numerical values, we may obtain

\begin{matrix} \frac{\partial J_{3} (v)}{\partial v} & = & \frac{\partial J_{2} (v)}{\partial v} + \frac{\partial J_{5} (v)}{\partial v} \\ \approx & \frac{\partial J_{2} (v)}{\partial v} + O (\pm 0.3), \end{matrix}

(26)

and

\begin{matrix} \frac{\partial^{2} J_{3} (v)}{\partial v^{2}} & = & \frac{\partial^{2} J_{2} (v)}{\partial v^{2}} + \frac{\partial^{2} J_{5} (v)}{\partial v^{2}} \\ \approx & \frac{\partial^{2} J_{2} (v)}{\partial v^{2}} + O (7.0) . \end{matrix}

(27)

Remark 6.1.

This new functional

J_{1}

has a relevant improvement in the convexity conditions concerning the previous functional J.

Indeed, we have obtained a gain in positiveness for the second variation

\frac{\partial^{2} J_{2} (v)}{\partial v^{2}},

which has increased of order

O (5 - 14) .

Moreover the difference between the approximate and exact equation

\frac{\partial J_{2} (v)}{\partial v} = 0

is of order

O (\pm 0.3)

which for appropriate parameters

γ > 0, α > 0

and

β > 0

, corresponds to a small perturbation in the original equation. Summarizing, the exact equation may be approximately solved in an appropriate sense.

Finally, for this last example, we highlight it is relatively easy to improve even more both such an approximation quality and the convexity conditions concerning the original variational model.

With such statements and results in mind, we may prove the following theorem.

Theorem 6.2.

Suppose

γ > 0,

α > 0

and

β > 0

are such that

\frac{\partial^{2} J_{3} (v)}{\partial v^{2}} > 0,

U_{3}

Assume also,

v_{0} \in U_{3}

is such that

δ J_{3} (v_{0}) = 0 .

Under such hypotheses,

J_{3}

is convex on

U_{3}

so that

J_{3} (v_{0}) = min_{v \in U_{3}} J_{3} (v) .

Moreover,

δ J (u_{0}) = 0 + O (\pm 0.3),

where

u_{0} = 10 v_{0} \in V_{1}

Proof.

From the hypotheses

\frac{\partial^{2} J_{3} (v)}{\partial v^{2}} > 0

U_{3}

, so that

J_{3}

is convex on the convex set

U_{3} .

Consequently, since

δ J_{3} (v_{0}) = 0

, we obtain

J_{3} (v_{0}) = min_{v \in U_{3}} J_{3} (v) .

Finally, from the approximation indicated in the last remark and

u_{0} \in V_{1}

we get

δ J (u_{0}) = 0 + O (\pm 0.3) .

The proof is complete.

□

7. An exact convex dual variational formulation for a non-convex primal one

In this section we develop a convex dual variational formulation suitable to compute a critical point for the corresponding primal one.

Let

Ω \subset R^{2}

be an open, bounded, connected set with a regular (Lipschitzian) boundary denoted by

\partial Ω .

Consider a functional

J : V \to R

where

J (u) = F (u_{x}, u_{y}) - {〈 u, f 〉}_{L^{2}},

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Here we denote

Y = Y^{*} = L^{2} (Ω)

and

Y_{1} = Y_{1}^{*} = L^{2} (Ω) \times L^{2} (Ω) .

Defining

V_{1} = {{u \in V : ∥ u ∥}_{1, \infty} \leq K_{1}}

for some appropriate

K_{1} > 0

, suppose also F is twice Fréchet differentiable and

det \{\frac{\partial^{2} F (u_{x}, u_{y})}{\partial v_{1} \partial v_{2}}\} \neq 0,

\forall u \in V_{1} .

Define now

F_{1} : V \to R

and

F_{2} : V \to R

F_{1} (u_{x}, u_{y}) = F (u_{x}, u_{y}) + \frac{ε}{2} \int_{Ω} u_{x}^{2} d x + \frac{ε}{2} \int_{Ω} u_{y}^{2} d x,

and

F_{2} (u_{x}, u_{y}) = \frac{ε}{2} \int_{Ω} u_{x}^{2} d x + \frac{ε}{2} \int_{Ω} u_{y}^{2} d x,

where here we denote

d x = d x_{1} d x_{2} .

Moreover, we define the respective Legendre transform functionals

F_{1}^{*}

and

F_{2}^{*}

F_{1}^{*} (v^{*}) = {〈 v_{1}, v_{1}^{*} 〉}_{L^{2}} + {〈 v_{2}, v_{2}^{*} 〉}_{L^{2}} - F_{1} (v_{1}, v_{2}),

where

v_{1}, v_{2} \in Y

are such that

v_{1}^{*} = \frac{\partial F_{1} (v_{1}, v_{2})}{\partial v_{1}},

v_{2}^{*} = \frac{\partial F_{1} (v_{1}, v_{2})}{\partial v_{2}},

and

F_{2}^{*} (v^{*}) = {〈 v_{1}, v_{1}^{*} + f_{1} 〉}_{L^{2}} + {〈 v_{2}, v_{2}^{*} 〉}_{L^{2}} - F_{2} (v_{1}, v_{2}),

where

v_{1}, v_{2} \in Y

are such that

v_{1}^{*} + f_{1} = \frac{\partial F_{2} (v_{1}, v_{2})}{\partial v_{1}},

v_{2}^{*} = \frac{\partial F_{2} (v_{1}, v_{2})}{\partial v_{2}} .

Here

f_{1}

is any function such that

{(f_{1})}_{x} = f, in Ω .

Furthermore, we define

\begin{matrix} J^{*} (v^{*}) & = & - F_{1}^{*} (v^{*}) + F_{2}^{*} (v^{*}) \\ = & - F_{1}^{*} (v^{*}) + \frac{1}{2 ε} \int_{Ω} {(v_{1}^{*} + f_{1})}^{2} d x + \frac{1}{2 ε} \int_{Ω} {(v_{2}^{*})}^{2} d x . \end{matrix}

(28)

Observe that through the target conditions

v_{1}^{*} + f_{1} = ε u_{x},

v_{2}^{*} = ε u_{y},

we may obtain the compatibility condition

{(v_{1}^{*} + f_{1})}_{y} - {(v_{2}^{*})}_{x} = 0 .

Define now

A^{*} = {v^{*} = (v_{1}^{*}, v_{2}^{*}) \in B_{r} (0, 0) \subset Y_{1}^{*} : {(v_{1}^{*} + f_{1})}_{y} - {(v_{2}^{*})}_{x} = 0, in Ω},

for some appropriate

r > 0

such that

J^{*}

is convex in

B_{r} (0, 0) .

Consider the problem of minimizing

J^{*}

subject to

v^{*} \in A^{*} .

Assuming

r > 0

is large enough so that the restriction in r is not active, at this point we define the associated Lagrangian

J_{1}^{*} (v^{*}, φ) = J^{*} (v^{*}) + {〈 φ, {(v_{1}^{*} + f)}_{y} - {(v_{2}^{*})}_{x} 〉}_{L^{2}},

where

φ

is an appropriate Lagrange multiplier.

Therefore

\begin{matrix} J_{1}^{*} (v^{*}) & = & - F_{1}^{*} (v^{*}) + \frac{1}{2 ε} \int_{Ω} {(v_{1}^{*} + f_{1})}^{2} d x + \frac{1}{2 ε} \int_{Ω} {(v_{2}^{*})}^{2} d x \\ + {〈 φ, {(v_{1}^{*} + f)}_{y} - {(v_{2}^{*})}_{x} 〉}_{L^{2}} . \end{matrix}

(29)

The optimal point in question will be a solution of the corresponding Euler-Lagrange equations for

J_{1}^{*} .

From the variation of

J_{1}^{*}

v_{1}^{*}

we obtain

- \frac{\partial F_{1}^{*} (v^{*})}{\partial v_{1}^{*}} + \frac{v_{1}^{*} + f}{ε} - \frac{\partial φ}{\partial y} = 0 .

(30)

From the variation of

J_{1}^{*}

v_{2}^{*}

we obtain

- \frac{\partial F_{1}^{*} (v^{*})}{\partial v_{2}^{*}} + \frac{v_{2}^{*}}{ε} + \frac{\partial φ}{\partial x} = 0 .

(31)

From the variation of

J_{1}^{*}

φ

we have

{(v_{1}^{*} + f)}_{y} - {(v_{2}^{*})}_{x} = 0 .

From this last equation, we may obtain

u \in V

such that

v_{1}^{*} + f = ε u_{x},

and

v_{2}^{*} = ε u_{y} .

From this and the previous extremal equations indicated we have

- \frac{\partial F_{1}^{*} (v^{*})}{\partial v_{1}^{*}} + u_{x} - \frac{\partial φ}{\partial y} = 0,

and

- \frac{\partial F_{1}^{*} (v^{*})}{\partial v_{2}^{*}} + u_{y} + \frac{\partial φ}{\partial x} = 0 .

so that

v_{1}^{*} + f = \frac{\partial F_{1} (u_{x} - φ_{y}, u_{y} + φ_{x})}{\partial v_{1}},

and

v_{2}^{*} = \frac{\partial F_{1} (u_{x} - φ_{y}, u_{y} + φ_{x})}{\partial v_{2}} .

From this and equation (30) and (31) we have

\begin{matrix} - ε {(\frac{\partial F_{1}^{*} (v^{*})}{\partial v_{1}^{*}})}_{x} - ε {(\frac{\partial F_{1}^{*} (v^{*})}{\partial v_{2}^{*}})}_{y} \\ + {(v_{1}^{*} + f_{1})}_{x} + {(v_{2}^{*})}_{y} \\ = & - ε u_{x x} - ε u_{y y} + {(v_{1}^{*})}_{x} + {(v_{2}^{*})}_{y} + f = 0 . \end{matrix}

(32)

Replacing the expressions of

v_{1}^{*}

and

v_{2}^{*}

into this last equation, we have

- ε u_{x x} - ε u_{y y} + {(\frac{\partial F_{1} (u_{x} - φ_{y}, u_{y} + φ_{x})}{\partial v_{1}})}_{x} + {(\frac{\partial F_{1} (u_{x} - φ_{y}, u_{y} + φ_{x})}{\partial v_{2}})}_{y} + f = 0,

so that

{(\frac{\partial F (u_{x} - φ_{y}, u_{y} + φ_{x})}{\partial v_{1}})}_{x} + {(\frac{\partial F (u_{x} - φ_{y}, u_{y} + φ_{x})}{\partial v_{2}})}_{y} + f = 0, in Ω .

(33)

Observe that if

\nabla^{2} φ = 0

then there exists

\hat{u}

such that u and

φ

are also such that

u_{x} - φ_{y} = {\hat{u}}_{x}

and

u_{y} + φ_{x} = {\hat{u}}_{y} .

The boundary conditions for

φ

must be such that

\hat{u} \in W_{0}^{1, 2} .

From this and equation (33) we obtain

δ J (\hat{u}) = 0 .

Summarizing, we may obtain a solution

\hat{u} \in W_{0}^{1, 2}

of equation

δ J (\hat{u}) = 0

by minimizing

J^{*}

A^{*}

Finally, observe that clearly

J^{*}

is convex in an appropriate large ball

B_{r} (0, 0)

for some appropriate

r > 0

8. Another primal dual formulation for a related model

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(34)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times Y^{*} \to R

\begin{matrix} J_{1}^{*} (u, v_{0}^{*}) & = & - \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x - {〈 u^{2}, v_{0}^{*} 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 v_{0}^{*} u - f)}^{2} d x + {〈 u, f 〉}_{L^{2}} \\ + \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x + β \int_{Ω} v_{0}^{*} d x, \end{matrix}

(35)

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+}

for some appropriate

K_{3} > 0

to be specified.

Moreover define

B^{*} = {v_{0}^{*} \in Y^{*} : ∥ v_{0}^{*} ∥_{\infty} \leq K}

for some appropriate

K > 0

to be specified.

Observe that, denoting

φ = - γ \nabla^{2} u + 2 v_{0}^{*} u - f

we have

\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial {(v_{0}^{*})}^{2}} = \frac{1}{α} + 4 K_{1} u^{2}

\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial u^{2}} = γ \nabla^{2} - 2 v_{0}^{*} + K_{1} {(- γ \nabla^{2} + 2 v_{0}^{*})}^{2}

and

\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial u \partial v_{0}^{*}} = K_{1} (2 φ + 2 (- γ \nabla^{2} u + 2 v_{0}^{*} u)) - 2 u

so that

\begin{matrix} det {δ^{2} J_{1}^{*} (u, v_{0}^{*})} \\ = & \frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial {(v_{0}^{*})}^{2}} \frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial u^{2}} - {(\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial u \partial v_{0}^{*}})}^{2} \\ = & \frac{K_{1} {(- γ \nabla^{2} + 2 v_{0}^{*})}^{2}}{α} - \frac{γ \nabla^{2} + 2 v_{0}^{*} + 4 α u^{2}}{α} \\ - 4 K_{1}^{2} φ^{2} - 8 K_{1} φ (- γ \nabla^{2} + 2 v_{0}^{*}) u + 8 K_{1} φ u \\ + 4 K_{1} (- γ \nabla^{2} u + 2 v_{0}^{*} u) u . \end{matrix}

(36)

Observe now that a critical point

φ = 0

and

(- γ \nabla^{2} u + 2 v_{0}^{*} u) u = f u \geq 0

Ω

Therefore, for an appropriate large

K_{1} > 0

, also at a critical point, we have

\begin{matrix} det {δ^{2} J_{1}^{*} (u, v_{0}^{*})} \\ = & 4 K_{1} f u - \frac{δ^{2} J (u)}{α} + K_{1} \frac{{(- γ \nabla^{2} + 2 v_{0}^{*})}^{2}}{α} > 0 . \end{matrix}

(37)

Remark 8.1.

From this last equation we may observe that

J_{1}^{*}

has a large region of convexity about any critical point

(u_{0}, {\hat{v}}_{0}^{*})

, that is, there exists a large

r > 0

such that

J_{1}^{*}

is convex on

B_{r} (u_{0}, {\hat{v}}_{0}^{*}) .

With such results in mind, we may easily prove the following theorem.

Theorem 8.2.

Assume

K_{1} ≫ max {1, K, K_{3}}

and suppose

(u_{0}, {\hat{v}}_{0}^{*}) \in V_{1} \times B^{*}

is such that

δ J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = 0 .

Under such hypotheses, there exists

r > 0

such that

J_{1}^{*}

is convex in

E^{*} = B_{r} (u_{0}, {\hat{v}}_{0}^{*}) \cap (V_{1} \times B^{*})

δ J (u_{0}) = 0,

and

- J (u_{0}) = J_{1} (u_{0}, {\hat{v}}_{0}^{*}) = inf_{(u, v_{0}^{*}) \in E^{*}} J_{1}^{*} (u, v_{0}^{*}) .

9. A third primal dual formulation for a related model

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(38)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times Y^{*} \times Y^{*} \to R

\begin{matrix} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{1}{2} \int_{Ω} K u^{2} d x \\ - {〈 u, v_{1}^{*} 〉}_{L^{2}} + \frac{1}{2} \int_{Ω} \frac{{(v_{1}^{*})}^{2}}{(- 2 v_{0}^{*} + K)} d x \\ + \frac{K_{1}}{2} \int_{Ω} {(v_{1}^{*} - (- γ \nabla^{2} u) - K u)}^{2} d x \\ \frac{1}{2} \int_{Ω} K_{2} (u - \frac{v_{1}^{*}}{(- 2 v_{0}^{*} + K)}) d x + d x + {〈 u, f 〉}_{L^{2}} \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x . \end{matrix}

(39)

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+}

for some appropriate

K_{3} > 0

to be specified.

Moreover define

B^{*} = {v_{0}^{*} \in Y^{*} : ∥ v_{0}^{*} ∥_{\infty} \leq K_{4}}

and

D^{*} = {v_{1}^{*} \in Y^{*} : ∥ v_{1}^{*} ∥ \leq K_{5}},

for some appropriate real constants

K_{4}, K_{5} > 0

to be specified.

Observe that we may obtain

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial {(v_{1}^{*})}^{2}} = K_{1} + \frac{1}{- 2 v_{0}^{*} + K} + \frac{K_{2}}{{(- 2 v_{0}^{*} + K)}^{2}}

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u^{2}} = - γ \nabla^{2} + K + K_{1} {(- γ \nabla^{2} + K)}^{2} + K_{2}

and

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}} = - 1 - K_{1} (- γ \nabla^{2} + K) - \frac{K_{2}}{- 2 v_{0}^{*} + K}

so that

\begin{matrix} det \{\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}}\} \\ = & \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial {(v_{1}^{*})}^{2}} \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u^{2}} - {(\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}})}^{2} \\ = & \frac{6 K_{1} (- γ \nabla^{2}) {(- γ \nabla^{2} + 2 v_{0}^{*})}^{2} + (- γ \nabla^{2} + 2 v_{0}^{*}) (- γ \nabla^{2} (- 2 v_{0}^{*}) K_{1} - v_{0} - 4 γ \nabla^{2})}{2 {(- γ \nabla^{2} + v_{0}^{*})}^{2}} \\ \equiv & H (v_{0}^{*}) . \end{matrix}

(40)

With such results in mind, we may easily prove the following theorem.

Theorem 9.1.

Assume

K = 2 γ \nabla^{2},

K_{2} = 10 (- γ \nabla^{2})

and

K_{1} ≫ max {1, | K |, K_{2}, K_{3}, K_{4}, K_{5}}

and suppose

(u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \in V_{1} \times D^{*} \times B^{*}

is such that

δ J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = 0

and

H ({\hat{v}}_{0}^{*}) > 0 .

Assume also

\frac{\partial^{2} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, v_{0}^{*})}{\partial {(v_{0}^{*})}^{2}} < 0,

\forall v_{0}^{*} \in B^{*} .

Under such hypotheses, we have that

δ J (u_{0}) = 0,

and

\begin{matrix} J (u_{0}) & = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(41)

Proof.

The proof that

δ J (u_{0}) = 0

and

J (u_{0}) = J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*})

may be easily made similarly as in the previous sections.

Moreover, from the hypotheses,

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, {\hat{v}}_{0}^{*})

and

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, v_{0}^{*}) .

From this, a standard saddle point theorem and the remaining hypotheses, we may infer that

\begin{matrix} J (u_{0}) & = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(42)

The proof is complete. □

10. A fourth primal dual formulation for a related model

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(43)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times Y^{*} \times Y^{*} \to R

\begin{matrix} J_{1}^{*} (u, v_{0}^{*}, v_{1}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{1}{2} \int_{Ω} K u^{2} d x \\ - {〈 u, v_{1}^{*} 〉}_{L^{2}} + \frac{1}{2} \int_{Ω} \frac{{(v_{1}^{*})}^{2}}{(- 2 v_{0}^{*} + K)} d x \\ + \frac{1}{2 (α + ε)} \int_{Ω} {(v_{0}^{*} - α (u^{2} - β))}^{2} d x + {〈 u, f 〉}_{L^{2}} \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x, \end{matrix}

(44)

where

ε > 0

is a small real constant.

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+}

for some appropriate

K_{3} > 0

to be specified.

Moreover define

B^{*} = {v_{0}^{*} \in Y^{*} : ∥ v_{0}^{*} ∥_{\infty} \leq K_{4}}

and

D^{*} = {v_{1}^{*} \in Y^{*} : ∥ v_{1}^{*} ∥ \leq K_{5}},

for some appropriate real constants

K_{4}, K_{5} > 0

to be specified.

Observe that, defining

φ = v_{0}^{*} - α (u^{2} - β)

we may obtain

\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*}, v_{1}^{*})}{\partial u^{2}} = - γ \nabla^{2} + K + \frac{α}{α + ε} 4 u^{2} - 2 φ \frac{α}{α + ε}

\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*}, v_{1}^{*})}{\partial {(v_{1}^{*})}^{2}} = \frac{1}{- 2 v_{0}^{*} + K}

and

\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*}, v_{1}^{*})}{\partial u \partial v_{1}^{*}} = - 1

so that

\begin{matrix} det \{\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*}, v_{1}^{*})}{\partial u \partial v_{0}^{*}}\} \\ = & \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial {(v_{1}^{*})}^{2}} \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u^{2}} - {(\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}})}^{2} \\ = & \frac{- γ \nabla^{2} + 2 v_{0}^{*} + 4 \frac{α^{2}}{α + ε} u^{2} - 2 \frac{α}{α + ε} φ}{- 2 v_{0}^{*} + K} \\ \equiv & H (u, v_{0}^{*}) . \end{matrix}

(45)

However, at a critical point, we have

φ = 0

so that, we define

C_{v_{0}^{*}}^{*} = {u \in V : φ \leq 0} .

From such results, assuming

K ≫ max {K_{3}, K_{4}, K_{5}},

define now

E_{v_{0}^{*}} = {u \in V : H (u, v_{0}^{*}) > 0} .

With such results in mind, we may easily prove the following theorem.

Theorem 10.1.

Suppose

(u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \in E^{*} = (V_{1} \cap C_{{\hat{v}}_{0}^{*}} \cap E_{{\hat{v}}_{0}^{*}}) \times D^{*} \times B^{*}

is such that

δ J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}, {\hat{v}}_{1}^{*}) = 0 .

Under such hypotheses, we have that

δ J (u_{0}) = 0

and

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} J (u) \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(46)

Proof.

The proof that

δ J (u_{0}) = 0

and

J (u_{0}) = J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*})

may be easily made similarly as in the previous sections.

Moreover, from the hypotheses, we have

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, {\hat{v}}_{0}^{*})

and

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, v_{0}^{*}) .

From this, from a standard saddle point theorem and the remaining hypotheses, we may infer that

\begin{matrix} J (u_{0}) & = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(47)

Moreover, observe that

\begin{matrix} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) & = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, {\hat{v}}_{0}^{*}) \\ \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{K}{2} \int_{Ω} u^{2} d x \\ + {〈 u^{2}, {\hat{v}}_{0}^{*} 〉}_{L^{2}} - \frac{K}{2} \int_{Ω} u^{2} d x \\ - \frac{1}{2 α} \int_{Ω} {({\hat{v}}_{0}^{*})}^{2} d x - β \int_{Ω} {\hat{v}}_{0}^{*} d x \\ + \frac{1}{2 (α + ε)} \int_{Ω} {({\hat{v}}_{0}^{*} - α (u^{2} - β))}^{2} d x - {〈 u, f 〉}_{L^{2}} \\ \leq & sup_{v_{0}^{*} \in Y^{*}} \{\frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + 〈 u^{2}, v_{0}^{*} 〉 \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x \\ + \frac{1}{2 (α + ε)} \int_{Ω} {(v_{0}^{*} - α (u^{2} - β))}^{2} d x - {〈 u, f 〉}_{L^{2}}\} \\ = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \forall u \in V_{1} . \end{matrix}

(48)

Summarizing, we have got

J (u_{0}) = J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \leq inf_{u \in V_{1}} J (u) .

From such results, we may infer that

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} J (u) \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(49)

The proof is complete. □

11. A fifth primal dual formulation for a related model

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(50)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times Y^{*} \to R

\begin{matrix} J_{1}^{*} (u, v_{0}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x - {〈 u^{2}, v_{0}^{*} 〉}_{L^{2}} \\ + \frac{1}{2 (α + ε)} \int_{Ω} {(v_{0}^{*} - α (u^{2} - β))}^{2} d x - {〈 u, f 〉}_{L^{2}} \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x, \end{matrix}

(51)

where

ε > 0

is a small real constant.

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+}

for some appropriate real constant

K_{3} > 0

Moreover define

B^{*} = {v_{0}^{*} \in Y^{*} : ∥ v_{0}^{*} ∥_{\infty} \leq K_{4}}

for some appropriate real constant

K_{4} > 0

Observe that, denoting

φ = v_{0}^{*} - α (u^{2} - β)

, we may obtain

\begin{matrix} \frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial u^{2}} & = & - γ \nabla^{2} + 2 v_{0} \\ + \frac{α^{2}}{α + ε} 4 u^{2} - 2 \frac{φ}{α + ε} α \\ \equiv & H (u, v_{0}^{*}), \end{matrix}

(52)

and

\frac{\partial^{2} J_{1}^{*} (u, v_{0}^{*})}{\partial {(v_{0}^{*})}^{2}} = - \frac{1}{α} + \frac{1}{α + ε} < 0

However, at a critical point, we have

φ = 0

so that, we define

C_{v_{0}^{*}}^{*} = {u \in V : φ \leq 0} .

Define also,

E_{v_{0}^{*}} = {u \in V : H (u, v_{0}^{*}) > 0} .

With such results in mind, we may easily prove the following theorem.

Theorem 11.1.

Suppose

(u_{0}, {\hat{v}}_{0}^{*}) \in E^{*} = (V_{1} \cap C_{{\hat{v}}_{0}^{*}} \cap E_{{\hat{v}}_{0}^{*}}) \times B^{*}

is such that

δ J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = 0 .

Under such hypotheses, we have that

δ J (u_{0}) = 0

and

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} J (u) \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & inf_{u \in V_{1}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{1}^{*} (u, v_{0}^{*})\} . \end{matrix}

(53)

Proof.

The proof that

δ J (u_{0}) = 0

and

J (u_{0}) = J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*})

may be easily made similarly as in the previous sections.

Moreover, from the hypotheses, we have

J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = inf_{u \in V_{1}} J_{1}^{*} (u, {\hat{v}}_{0}^{*})

and

J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u_{0}, v_{0}^{*}) .

From this, from a standard saddle point theorem and the remaining hypotheses, we may infer that

\begin{matrix} J (u_{0}) & = & J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & inf_{u \in V_{1}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{1}^{*} (u, v_{0}^{*})\} . \end{matrix}

(54)

Moreover, observe that

\begin{matrix} J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) & = & inf_{u \in V_{1}} J_{1}^{*} (u, {\hat{v}}_{0}^{*}) \\ \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈 u^{2}, {\hat{v}}_{0}^{*} 〉}_{L^{2}} \\ - \frac{1}{2 α} \int_{Ω} {({\hat{v}}_{0}^{*})}^{2} d x - β \int_{Ω} {\hat{v}}_{0}^{*} d x \\ + \frac{1}{2 (α + ε)} \int_{Ω} {({\hat{v}}_{0}^{*} - α (u^{2} - β))}^{2} d x - {〈 u, f 〉}_{L^{2}} \\ \leq & sup_{v_{0}^{*} \in Y^{*}} \{\frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + 〈 u^{2}, v_{0}^{*} 〉 \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x \\ + \frac{1}{2 (α + ε)} \int_{Ω} {(v_{0}^{*} - α (u^{2} - β))}^{2} d x - {〈 u, f 〉}_{L^{2}}\} \\ = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \forall u \in V_{1} . \end{matrix}

(55)

Summarizing, we have got

J (u_{0}) = J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \leq inf_{u \in V_{1}} J (u) .

From such results, we may infer that

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} J (u) \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & inf_{u \in V_{1}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{1}^{*} (u, v_{0}^{*})\} . \end{matrix}

(56)

The proof is complete. □

12. One more primal dual formulation for a related model

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(57)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times Y^{*} \times Y^{*} \to R

\begin{matrix} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{K}{2} \int_{Ω} u^{2} d x - {〈 u, v_{1}^{*} 〉}_{L^{2}} \\ + \frac{1}{2} \int_{Ω} \frac{{(v_{1}^{*})}^{2}}{- 2 v_{0}^{*} + K} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K^{2}}{2} \int_{Ω} {(\frac{v_{1}^{*} + f}{- γ \nabla^{2} + K} - \frac{v_{1}^{*}}{- 2 v_{0}^{*} + K})}^{2} d x \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x, \end{matrix}

(58)

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+}

specifically for a constant

K_{3} = \sqrt{\frac{1}{4 α}}

Moreover define

B^{*} = {v_{0}^{*} \in Y^{*} : ∥ v_{0}^{*} ∥_{\infty} \leq K_{4}}

and

D^{*} = {v_{1}^{*} \in Y^{*} : ∥ v_{1}^{*} ∥_{\infty} \leq K_{5}}

for some appropriate real constants

K_{4} > 0

and

K_{5} > 0

Observe that

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u^{2}} = - γ \nabla^{2} + K,

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial {(v_{1}^{*})}^{2}} = \frac{1}{- 2 v_{0}^{*} + K} + \frac{K^{2} {(- γ \nabla^{2} + 2 v_{0}^{*})}^{2}}{{[(- γ \nabla^{2} + K) (- 2 v_{0}^{*} + K)]}^{2}},

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}} = - 1,

so that

\begin{matrix} det (\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}}) & = & \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial {(v_{1}^{*})}^{2}} \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u^{2}} - {(\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}})}^{2} \\ = & O (\frac{K^{2} (2 (- γ \nabla^{2} + 2 v_{0}^{*}) + 2 {(- γ \nabla^{2} + 2 v_{0}^{*})}^{2})}{(- γ \nabla^{2} + K) {(- 2 v_{0}^{*} + K)}^{2}}) \\ \equiv & H (v_{0}^{*}) . \end{matrix}

(59)

With such results in mind, we may easily prove the following theorem.

Theorem 12.1.

Assume

K ≫ max {K_{3}, K_{4}, K_{5}, 1}

and suppose

(u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \in V_{1} \times D^{*} \times B^{*}

is such that

δ J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = 0 .

Suppose also

H ({\hat{v}}_{0}^{*}) > 0 .

Under such hypotheses, we have that

δ J (u_{0}) = 0

and

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{K^{2}}{2} \int_{Ω} {(\frac{(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}{- γ \nabla^{2} + K})}^{2} d x\} \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u,, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(60)

Proof.

The proof that

δ J (u_{0}) = - γ \nabla^{2} u_{0} + 2 α (u^{2} - β) u_{0} - f = 0,

{\hat{v}}_{0}^{*} = α (u_{0}^{2} - β)

and

J (u_{0}) = J (u_{0}) + \frac{K^{2}}{2} \int_{Ω} {(\frac{(- γ \nabla^{2} u_{0} + 2 {\hat{v}}_{0}^{*} u_{0} - f)}{- γ \nabla^{2} + K})}^{2} d x = J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*})

may be easily made similarly as in the previous sections.

Moreover, from the hypotheses, we have

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, {\hat{v}}_{0}^{*})

and

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, v_{0}^{*}) .

From this, from a standard saddle point theorem and the remaining hypotheses, we may infer that

\begin{matrix} J (u_{0}) & = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(61)

Moreover, observe that

\begin{matrix} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) & = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, {\hat{v}}_{0}^{*}) \\ \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈 u^{2}, {\hat{v}}_{0}^{*} 〉}_{L^{2}} \\ - \frac{1}{2 α} \int_{Ω} {({\hat{v}}_{0}^{*})}^{2} d x - β \int_{Ω} {\hat{v}}_{0}^{*} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K^{2}}{2} \int_{Ω} {(\frac{(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}{- γ \nabla^{2} + K})}^{2} d x \\ \leq & sup_{v_{0}^{*} \in Y^{*}} \{\frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + 〈 u^{2}, v_{0}^{*} 〉 \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K^{2}}{2} \int_{Ω} {(\frac{(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}{- γ \nabla^{2} + K})}^{2} d x\} \\ = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}} \\ + \frac{K^{2}}{2} \int_{Ω} {(\frac{(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}{- γ \nabla^{2} + K})}^{2} d x, \forall u \in V_{1} . \end{matrix}

(62)

From this we have got

\begin{matrix} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) & \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K^{2}}{2} \int_{Ω} {(\frac{(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}{- γ \nabla^{2} + K})}^{2} d x, \forall u \in V_{1} . \end{matrix}

(63)

Therefore, from such results we may obtain

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{K^{2}}{2} \int_{Ω} {(\frac{(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}{- γ \nabla^{2} + K})}^{2} d x\} \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u,, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(64)

The proof is complete. □

13. A convex primal dual formulation for a related model

In this section we present a convex primal dual formulation.

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(65)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times Y^{*} \times Y^{*} \to R

\begin{matrix} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{K}{2} \int_{Ω} u^{2} d x - {〈 u, v_{1}^{*} 〉}_{L^{2}} \\ + \frac{1}{2} \int_{Ω} \frac{{(v_{1}^{*})}^{2}}{- 2 v_{0}^{*} + K} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(v_{1}^{*} + f - (- γ \nabla^{2} + K) u)}^{2} d x + \frac{K_{1}}{8 K_{3}^{2}} \int_{Ω} {(v_{1}^{*} - (- 2 v_{0}^{*} + K) u)}^{2} d x \\ - \frac{K_{1}}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x . \end{matrix}

(66)

for appropriate real constants

K_{1} > 0, K_{3} > 0

to be specified.

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+} .

Moreover define

B^{*} = {v_{0}^{*} \in Y^{*} : ∥ v_{0}^{*} ∥_{\infty} \leq K_{4}}

and

D^{*} = {v_{1}^{*} \in Y^{*} : ∥ v_{1}^{*} ∥_{\infty} \leq K_{5}}

for some appropriate real constants

K_{4} > 0

and

K_{5} > 0

Observe that, denoting

φ = v_{0}^{*} - α (u^{2} - β),

we have

\begin{matrix} \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u^{2}} & = & (- γ \nabla^{2} + K) + K_{1} {(- γ \nabla^{2} + K)}^{2} + \frac{K_{1}}{4 K_{3}^{2}} {(- 2 v_{0}^{*} + K)}^{2} \\ - 4 K_{1} α^{2} u^{2} + K_{1} φ (2 α), \end{matrix}

(67)

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial {(v_{1}^{*})}^{2}} = \frac{1}{- 2 v_{0}^{*} + K} + K_{1} + \frac{K_{1}}{4 K_{3}^{2}},

\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}} = - 1 - K_{1} (- γ \nabla^{2} + K) - \frac{K_{1}}{4 K_{3}^{2}} (- 2 v_{0}^{*} + K),

so that

\begin{matrix} det (\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}}) & = & \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial {(v_{1}^{*})}^{2}} \frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u^{2}} - {(\frac{\partial^{2} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})}{\partial u \partial v_{1}^{*}})}^{2} \\ = & O (K_{1}^{2} K^{2}) \\ > & 0, in V_{1} \times D^{*} \times B^{*} . \end{matrix}

(68)

With such results in mind, we may easily prove the following theorem.

Theorem 13.1.

Assume

K_{1} ≫ K ≫ max {K_{3}, K_{4}, K_{5}, 1}

and suppose

(u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \in V_{1} \times D^{*} \times B^{*}

is such that

δ J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = 0 .

Under such hypotheses, we have that

δ J (u_{0}) = 0

and

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}^{2} d x\} \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u,, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(69)

Proof.

The proof that

δ J (u_{0}) = - γ \nabla^{2} u_{0} + 2 α (u^{2} - β) u_{0} - f = 0,

{\hat{v}}_{0}^{*} = α (u_{0}^{2} - β)

and

J (u_{0}) = J (u_{0}) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u_{0} + 2 {\hat{v}}_{0}^{*} u_{0} - f)}^{2} d x = J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*})

may be easily made similarly as in the previous sections.

Moreover, from the hypotheses and from the above lines, we have that

J_{1}^{*}

is convex in

(u, v_{1}^{*})

and concave in

v_{0}^{*}

V_{1} \times D^{*} \times B^{*}

, so that

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, {\hat{v}}_{0}^{*})

and

J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) = sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, v_{0}^{*}) .

From this, from the standard Min-Max theorem and the remaining hypotheses, we may infer that

\begin{matrix} J (u_{0}) & = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(70)

Moreover, observe that

\begin{matrix} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) & = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, {\hat{v}}_{0}^{*}) \\ \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈 u^{2}, {\hat{v}}_{0}^{*} 〉}_{L^{2}} \\ - \frac{1}{2 α} \int_{Ω} {({\hat{v}}_{0}^{*})}^{2} d x - β \int_{Ω} {\hat{v}}_{0}^{*} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}^{2} d x \\ \leq & sup_{v_{0}^{*} \in Y^{*}} \{\frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + 〈 u^{2}, v_{0}^{*} 〉 \\ - \frac{1}{2 α} \int_{Ω} {(v_{0}^{*})}^{2} d x - β \int_{Ω} v_{0}^{*} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}^{2} d x\} \\ = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}^{2} d x, \forall u \in V_{1} . \end{matrix}

(71)

From this we have got

\begin{matrix} J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) & \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}^{2} d x, \forall u \in V_{1} . \end{matrix}

(72)

Therefore, from such results we may obtain

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u - f)}^{2} d x\} \\ = & J_{1}^{*} (u_{0}, {\hat{v}}_{1}^{*}, {\hat{v}}_{0}^{*}) \\ = & inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} \{sup_{v_{0}^{*} \in B^{*}} J_{1}^{*} (u,, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{(u, v_{1}^{*}) \in V_{1} \times D^{*}} J_{1}^{*} (u, v_{1}^{*}, v_{0}^{*})\} . \end{matrix}

(73)

The proof is complete. □

14. Another primal dual formulation for a related model

In this section we present another primal dual formulation.

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(74)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times Y^{*} \to R

\begin{matrix} J_{1}^{*} (u, v_{0}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈 u^{2}, v_{0}^{*} 〉}_{L^{2}} \\ + \frac{α - ε}{2} \int_{Ω} u^{4} d x - {〈 u, f 〉}_{L^{2}} \\ - \frac{1}{2 ε} \int_{Ω} {(v_{0}^{*} + α β)}^{2} d x, \end{matrix}

(75)

and

J_{2}^{*} : V \times Y^{*} \to R,

\begin{matrix} J_{2}^{*} (u, v_{0}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈 u^{2}, v_{0}^{*} 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 v_{0}^{*} u - 2 (α - ε) u^{3} - f)}^{2} d x \\ + \frac{α - ε}{2} \int_{Ω} u^{4} d x - {〈 u, f 〉}_{L^{2}} \\ - \frac{1}{2 ε} \int_{Ω} {(v_{0}^{*} + α β)}^{2} d x, \end{matrix}

(76)

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+} .

Moreover define

B^{*} = {v_{0}^{*} \in Y^{*} : ∥ v_{0}^{*} ∥_{\infty} \leq K_{4}}

for some appropriate constants

K_{3} > 0

and

K_{4} > 0

Observe that, for

K_{1} = 1 / \sqrt{ε},

we have

\begin{matrix} \frac{\partial^{2} J_{2}^{*} (u, v_{0}^{*})}{\partial u^{2}} & = & (- γ \nabla^{2} + 2 v_{0}^{*} + 6 (α - ε) u^{2}) + K_{1} {(- γ \nabla^{2} + 2 v_{0}^{*} + 6 (α - ε) u^{2})}^{2} \\ + K_{1} (- γ \nabla^{2} u + 2 v_{0}^{*} u + 2 (α - ε) u^{3} - f) 12 (α - ε) u d x, \end{matrix}

(77)

\begin{matrix} \frac{\partial^{2} J_{2}^{*} (u, v_{0}^{*})}{\partial {(v_{0}^{*})}^{2}} & = & K_{1} 4 u^{2} - \frac{1}{ε} \\ < & 0, \forall u \in V_{1}, v_{0}^{*} \in B^{*} . \end{matrix}

(78)

Define now

A_{2} (u, v_{0}^{*}) = (- γ \nabla^{2} u + 2 v_{0}^{*} u + 2 (α - ε) u^{3} - f) 12 (α - ε) u,

C^{*} = {(u, v_{0}^{*}) \in V \times B^{*} : {∥ A_{2} (u, v_{0}^{*}) ∥}_{\infty} \leq ε_{1}

for a small real parameter

ε_{1} > 0 .

Finally, define

E_{v_{0}^{*}} = \{u \in V : \frac{\partial^{2} A_{2} (u, v_{0}^{*})}{\partial u^{2}} > 0\} .

Observe that

E_{v_{0}^{*}} \cap V_{1}

is a convex set,

\forall v_{0}^{*} \in B^{*}

(for the proof of a similar result please see Theorem 8.7.1 at pages 297, 298 and 299 in [5]).)

With such results in mind, we may easily prove the following theorem.

Theorem 14.1.

Assume

K_{1} ≫ 1 ≫ ε_{1}

and suppose

(u_{0}, {\hat{v}}_{0}^{*}) \in V_{1} \times B^{*}

is such that

δ J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = 0

and

u_{0} \in E_{{\hat{v}}_{0}^{*}} .

Under such hypotheses, we have that

δ J (u_{0}) = 0

and

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u + 2 (α - ε) u^{3} - f)}^{2} d x\} \\ = & J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{2}^{*} (u, v_{0}^{*})\} . \end{matrix}

(79)

Proof.

The proof that

δ J (u_{0}) = - γ \nabla^{2} u_{0} + 2 α (u^{2} - β) u_{0} - f = 0,

and

J (u_{0}) = J (u_{0}) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u_{0} + 2 {\hat{v}}_{0}^{*} u_{0} + 2 (α - ε) u_{0}^{3} - f)}^{2} d x = J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*})

may be easily made similarly as in the previous sections.

Moreover, from the hypotheses and from the above lines, since

J_{2}^{*}

is concave in

v_{0}^{*}

V_{1} \times B^{*}

and

u_{0} \in E_{{\hat{v}}_{0}^{*}},

we have that

J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = inf_{u \in V_{1}} J_{2}^{*} (u, {\hat{v}}_{0}^{*})

and

J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = sup_{v_{0}^{*} \in B^{*}} J_{2}^{*} (u_{0}, v_{0}^{*}) .

From this, from the standard Saddle Point Theorem and the remaining hypotheses, we may infer that

\begin{matrix} J (u_{0}) & = & J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & inf_{u \in V_{1}} \{sup_{v_{0}^{*} \in B^{*}} J_{2}^{*} (u, v_{1}^{*}, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{2}^{*} (u, v_{0}^{*})\} . \end{matrix}

(80)

Moreover, observe that

\begin{matrix} J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) & = & inf_{u \in V_{1}} J_{2}^{*} (u, {\hat{v}}_{0}^{*}) \\ \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈 u^{2}, {\hat{v}}_{0}^{*} 〉}_{L^{2}} + \frac{α - ε}{2} \int_{Ω} u^{4} d x \\ - \frac{1}{2 ε} \int_{Ω} {({\hat{v}}_{0}^{*} + α β)}^{2} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u + 2 (α - ε) u^{3} - f)}^{2} d x \\ \leq & sup_{v_{0}^{*} \in Y^{*}} \{\frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈 u^{2}, v_{0}^{*} 〉}_{L^{2}} + \frac{α - ε}{2} \int_{Ω} u^{4} d x \\ - \frac{1}{2 ε} \int_{Ω} {(v_{0}^{*} + α β)}^{2} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u + 2 (α - ε) u^{3} - f)}^{2} d x\} \\ = & J (u) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u + 2 (α - ε) u^{3} - f)}^{2} d x, \forall u \in V_{1} . \end{matrix}

(81)

From this we have got

\begin{matrix} J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) & \leq & J (u) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u + 2 (α - ε) u^{3} - f)}^{2} d x, \forall u \in V_{1} . \end{matrix}

(82)

Therefore, from such results we may obtain

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{K_{1}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 {\hat{v}}_{0}^{*} u + 2 (α - ε) u^{3} - f)}^{2} d x\} \\ = & J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{2}^{*} (u, v_{0}^{*})\} . \end{matrix}

(83)

The proof is complete. □

15. A convex primal dual formulation for a related model

In this section we present another convex primal dual formulation for the model in question.

Let

Ω \subset R^{3}

be an open, bounded and connected set with a regular boundary denoted by

\partial Ω .

Consider the functional

J : V \to R

where

\begin{matrix} J (u) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + \frac{α}{2} \int_{Ω} {(u^{2} - β)}^{2} d x \\ - {〈 u, f 〉}_{L^{2}}, \end{matrix}

(84)

α > 0, β > 0, γ > 0

V = W_{0}^{1, 2} (Ω)

and

f \in L^{2} (Ω) .

Let

N \in N

be a large natural number.

Denoting

Y = Y^{*} = L^{2} (Ω)

, define now

J_{1}^{*} : V \times {[Y^{*}]}^{N} \to R

\begin{matrix} J_{1}^{*} (u, v_{0}^{*}) & = & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈u^{2}, \sum_{j = 1}^{N} {(v_{0}^{*})}_{j}〉}_{L^{2}} \\ - {〈 u, f 〉}_{L^{2}} \\ - \frac{N}{2 α} \sum_{j = 1}^{N} \int_{Ω} {(v_{0}^{*})}_{j}^{2} d x - β \sum_{j = 1}^{N} \int_{Ω} {(v_{0}^{*})}_{j} d x, \end{matrix}

(85)

and

J_{2}^{*} : V \times {[Y^{*}]}^{N} \to R,

\begin{matrix} J_{2}^{*} (u, v_{0}^{*}) & = & J_{1}^{*} (u, v_{0}^{*}) + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + \sum_{j = 1}^{N} 2 {(v_{0}^{*})}_{j} u - f)}^{2} d x, \end{matrix}

(86)

Define also

A^{+} = {u \in V : u f \geq 0, a . e . in Ω},

V_{2} = {{u \in V : ∥ u ∥}_{\infty} \leq K_{3}},

and

V_{1} = V_{2} \cap A^{+} .

Moreover define

B^{*} = {v_{0}^{*} = {{(v_{0})}_{j}^{*}} \in {[Y^{*}]}^{N} : ∥ v_{0}^{*} ∥_{\infty} \leq K_{4}}

for some appropriate constants

K_{3} > 0

and

K_{4} > 0

Observe that,

\begin{matrix} \frac{\partial^{2} J_{2}^{*} (u, v_{0}^{*})}{\partial u^{2}} & = & \sqrt[4]{N} {(- γ \nabla^{2} + 2 \sum_{j = 1}^{N} {(v_{0}^{*})}_{j})}^{2} + (- γ \nabla^{2} + 2 \sum_{j = 1}^{N} {(v_{0}^{*})}_{j}) \\ > & 0, in V_{1} \cap B^{*}, \end{matrix}

(87)

and,

\begin{matrix} \frac{\partial^{2} J_{2}^{*} (u, v_{0}^{*})}{\partial {(v_{0}^{*})}^{2}} & < & 0, in V_{1} \times B^{*} . \end{matrix}

(88)

With such results in mind, we may easily prove the following theorem.

Theorem 15.1.

Assume

(u_{0}, {\hat{v}}_{0}^{*}) \in V_{1} \times B^{*}

is such that

δ J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = 0 .

Under such hypotheses, we have that

δ J (u_{0}) = 0

and

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} - f)}^{2} d x\} \\ = & J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{2}^{*} (u, v_{0}^{*})\} . \end{matrix}

(89)

Proof.

The proof that

δ J (u_{0}) = - γ \nabla^{2} u_{0} + 2 α (u^{2} - β) u_{0} - f = 0,

and

J (u_{0}) = J (u_{0}) + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u_{0} + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} u_{0} - f)}^{2} d x = J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*})

may be easily made similarly as in the previous sections.

Moreover, from the hypotheses and from the above lines, since

J_{2}^{*}

is concave in

v_{0}^{*}

and convex in u on

V_{1} \times B^{*}

, we have that

J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = inf_{u \in V_{1}} J_{2}^{*} (u, {\hat{v}}_{0}^{*})

and

J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) = sup_{v_{0}^{*} \in B^{*}} J_{2}^{*} (u_{0}, v_{0}^{*}) .

From this, from the standard Min-Max Theorem and the remaining hypotheses, we may infer that

\begin{matrix} J (u_{0}) & = & J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & inf_{u \in V_{1}} \{sup_{v_{0}^{*} \in B^{*}} J_{2}^{*} (u, v_{0}^{*})\} \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{2}^{*} (u, v_{0}^{*})\} . \end{matrix}

(90)

Moreover, by convexity in u,

\begin{matrix} J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) & = & inf_{u \in V_{1}} J_{2}^{*} (u, {\hat{v}}_{0}^{*}) \\ \leq & \frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈u^{2}, \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j}〉}_{L^{2}} \\ - \frac{N}{2 α} \sum_{j = 1}^{N} \int_{Ω} {({\hat{v}}_{0}^{*})}_{j}^{2} d x - β \sum_{j = 1}^{N} \int_{Ω} {({\hat{v}}_{0}^{*})}_{j} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} u - f)}^{2} d x \\ \leq & sup_{v_{0}^{*} \in {[Y^{*}]}^{N}} \{\frac{γ}{2} \int_{Ω} \nabla u \cdot \nabla u d x + {〈u^{2}, \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j}〉}_{L^{2}} \\ - \frac{N}{2 α} \sum_{j = 1}^{N} \int_{Ω} {({\hat{v}}_{0}^{*})}_{j}^{2} d x - β \sum_{j = 1}^{N} \int_{Ω} {(v_{0}^{*})}_{j} d x - {〈 u, f 〉}_{L^{2}} \\ + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} u - f)}^{2} d x\} \\ = & J (u) + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} u - f)}^{2} d x, \forall u \in V_{1} . \end{matrix}

(91)

From this we have got

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} u - f)}^{2} d x\} \\ = & J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{2}^{*} (u, v_{0}^{*})\} . \end{matrix}

(92)

\begin{matrix} J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) & \leq & J (u) + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} u - f)}^{2} d x, \forall u \in V_{1} . \end{matrix}

(93)

Therefore, from such results we may obtain

\begin{matrix} J (u_{0}) & = & inf_{u \in V_{1}} \{J (u) + \frac{\sqrt[4]{N}}{2} \int_{Ω} {(- γ \nabla^{2} u + 2 \sum_{j = 1}^{N} {({\hat{v}}_{0}^{*})}_{j} u - f)}^{2} d x\} \\ = & J_{2}^{*} (u_{0}, {\hat{v}}_{0}^{*}) \\ = & sup_{v_{0}^{*} \in B^{*}} \{inf_{u \in V_{1}} J_{2}^{*} (u, v_{0}^{*})\} . \end{matrix}

(94)

The proof is complete. □

References

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W.R. Bielski and J.J. Telega, A Contribution to Contact Problems for a Class of Solids and Structures, Arch. Mech., 37, 4-5, pp. 303-320, Warszawa 1985.
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F.S. Botelho, Variational Convex Analysis, Ph.D. thesis, Virginia Tech, Blacksburg, VA -USA, (2009).
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F. Botelho, Existence of solution for the Ginzburg-Landau system, a related optimal control problem and its computation by the generalized method of lines, Applied Mathematics and Computation, 218, 11976-11989, (2012). [CrossRef]
F. Botelho, Functional Analysis and Applied Optimization in Banach Spaces, Springer Switzerland, 2014. [CrossRef]
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L.D. Landau and E.M. Lifschits, Course of Theoretical Physics, Vol. 5- Statistical Physics, part 1. (Butterworth-Heinemann, Elsevier, reprint 2008).
R.T. Rockafellar, Convex Analysis, Princeton Univ. Press, (1970).
J.J. Telega, On the complementary energy principle in non-linear elasticity. Part I: Von Karman plates and three dimensional solids, C.R. Acad. Sci. Paris, Serie II, 308, 1193-1198; Part II: Linear elastic solid and non-convex boundary condition. Minimax approach, ibid, pp. 1313-1317 (1989).
A.Galka and J.J.Telega Duality and the complementary energy principle for a class of geometrically non-linear structures. Part I. Five parameter shell model; Part II. Anomalous dual variational priciples for compressed elastic beams, Arch. Mech. 47 (1995) 677-698, 699-724.
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Figure 1. solution

u_{0} (x)

for the case

f (x) = 0

Figure 1. solution

u_{0} (x)

for the case

f (x) = 0

Figure 2. solution

u_{0} (x)

for the case

f (x) = sin (π x) / 2

Figure 2. solution

u_{0} (x)

for the case

f (x) = sin (π x) / 2

Figure 3. solution

u_{0} (x)

for the case

f (x) = 0

Figure 3. solution

u_{0} (x)

for the case

f (x) = 0

Figure 4. solution

u_{0} (x)

for the case

f (x) = sin (π x) / 2

Figure 4. solution

u_{0} (x)

for the case

f (x) = sin (π x) / 2

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Duality Principles and Numerical Procedures for a Large Class of Non-convex Models in the Calculus of Variations

Abstract

1. Introduction

2. A general duality principle non-convex optimization

3. Another duality principle for a simpler related model in phase transition with a respective numerical example

4. A convex dual variational formulation for a third similar model

4.1. The algorithm through which we have obtained the numerical results

5. An improvement of the convexity conditions for a non-convex related model through an approximate primal formulation

6. An approximate convex variational formulation for another related model

7. An exact convex dual variational formulation for a non-convex primal one

8. Another primal dual formulation for a related model

9. A third primal dual formulation for a related model

10. A fourth primal dual formulation for a related model

11. A fifth primal dual formulation for a related model

12. One more primal dual formulation for a related model

13. A convex primal dual formulation for a related model

14. Another primal dual formulation for a related model

15. A convex primal dual formulation for a related model

References

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