1. Introduction
Let
be a complex Hilbert space and
A be a possibly unbounded self-adjoint operator defined on domain
. For
with
, define the uncertainty of
A at the point
h as
In 1929, Robertson [
11] derived the following mathematical form of the uncertainty principle of Heisenberg derived in 1927 [
7]. Recall that for two operators
and
, we define
and
.
Theorem 1.1.
[5,7,11,13] (Heisenberg-Robertson Uncertainty Principle) Let and be self-adjoint operators. Then for all with , we have
In 1930, Schrodinger improved Inequality (
1) [
12].
Theorem 1.2.
[12] (Heisenberg-Robertson-Schrodinger Uncertainty Principle) Let and be self-adjoint operators. Then for all with , we have
A fundamental drawback of Inequality (
1) is that if
h satisfies
, then the right side is zero. In 2014, Maccone and Pati derived the following two uncertainty principles which overturned this problem [
9].
Theorem 1.3.
[9] (Maccone-Pati Uncertainty Principle) Let and be self-adjoint operators. Then for all with , we have
Theorem 1.4.
[9] (Maccone-Pati Uncertainty Principle) Let and be self-adjoint operators. Then for all with , we have
In this note, we show that Theorem 1.3 can be generalized even for arbitrary maps on Lebesgue spaces using Clarkson inequalities. We also derive uncertainty principle for Banach spaces satisfying weak parallelogram law and Type-p Banach spaces.
Our main motivation comes from the sentence ‘The first proof, based on the parallelogram law, was communicated to us by an anonymous referee, while the second (independent) proof was our original argument” given in [
9]. Note that Clarkson inequalities are generalizations of Jordan-von Neumann parallelogram law in Hilbert space [
8].
2. Nonlinear Maccone-Pati Uncertainty Principle
We first define the uncertainty for maps on Lebesgue spaces. Let
be a subset and
be a map (need not be linear or Lipschitz). Given
and
, we define the uncertainty at
f relative to
a as
To derive our first uncertainty principle we need the following breakthrough inequalities of Clarkson.
Theorem 2.1.
[4,6,10] (Clarkson Inequalities) Let be a measure space.
Let and q be the conjugate index of p. Then
Let and q be the conjugate index of p. Then
Let and q be the conjugate index of p. Let and s be the conjugate index of r. Then
Theorem 2.2. (Nonlinear Maccone-Pati Uncertainty Principle) Let be a measure space. Let be subsets and , be maps. Let and .
and q be the conjugate index of p. Then
and q be the conjugate index of p. Then
Let and q be the conjugate index of p. Let and s be the conjugate index of r. Then
Proof. We prove (i) and remaining are similar. Using (i) in Theorem 2.1
□
We next note the following extension of Theorem 2.2 (we state generalizations of only (i) and (ii) and others are similar). Recall that the collection of all Lipschitz functions
satisfying
, denoted by
is a Banach space [
14] w.r.t. the Lipschitz norm
Corollary 2.1. Let be a measure space. Let be subsets and , be maps. Let and be such that .
In 1972, Bynum and Drew derived the following surprising result [
2].
Theorem 2.3 promoted the notion of parallelogram law spaces by Cheng and Ross [
3].
Definition 2.1.
[3] Let and . A Banach space is said to satisfy lower p-weak parallelogram law with constant C if
in this case, we write is p-LWP(C).
For parallelogram law spaces, by following a similar computation as in the proof of Theorem 2.2 we get the following theorem.
Theorem 2.4.
Let be p-LWP(C). Let be subsets and , be maps. Let and . Then
Corollary 2.2.
Let be p-LWP(C). Let be subsets and , be maps. Let and be such that . Then
We now proceed to derive nonlinear uncertainty principle for a class of Banach spaces (at present, we don’t know it for arbitrary Banach spaces). Recall that a Banach space
is said to be of Type-p,
[
1] if there exists a constant
satisfying following: For every
,
In this case, we define the Type-p constant of
as
Theorem 2.5.
Let be a Banach space of Type-p. Let be subsets and , be maps. Let and . Then
Proof. Using the definition of Type-p, we get
□
Corollary 2.3.
Let be a Banach space of Type-p. Let be subsets and , be maps. Let and be such that . Then
Note that we can derive following results if we won’t bother about power p.
Theorem 2.6.
Let be a Banach space. Let be subsets and , be maps. Let and . Then
Proof. By directly applying triangle inequality
□
Corollary 2.4.
Let be a Banach space. Let be subsets and , be maps. Let and be such that . Then
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