5.1. Periodic Boundary Conditions
Our starting point is the partition function of the one dimensional Ising model under periodic boundary conditions
(see Eq.(
17)) The RG approach is based on the (decimation) procedure known as the Kadanoff construction. It can be performed by a summing the configurations of every alternate spin variable in the chain and thus calculating of the partition function
. In this way one obtains a renormalized chain with only the half of number of the original
N spins (
N is supposed even). The essence of the RG approach is to obtain from the solution of the equation
new couplings
and
such that Eq.(
61) should take place.The normalized factor
is the contribution (per site) from the "decimated" spins. Unfortunately the Kadanoff procedure can be performed exactly only in the one-dimensional case [
30].
We will present a straightforward way of solving this problem using the recurrent equations of the Chebyshev polynomials.
First, we make use the substitution
in the Chebyshev polynomials which is a constituent of the partition function
in Eq.(
17) (and the physics of the system). Then we get
Note that this is a particular realization of semigroup functional equation (A11). In the case these analytic property the Chebyshev polynomials is intimately related to the physical notion of decimation procedure. Our aim is to clarify this connection. If
N is even,Eq.(
63) maps the Lee-Yang zeros for a system of size
N to the zeros of size
Let us introduce the partition function for the systems with
N and constant
,
and for the partition function obtained from the partial trace by summing over every second spin variable along the chain with new length
and and
respectively
Let us note that:
1.) The gist of of the proposed decimation transformation is to use the recursive relation between Chebyshev polynomials and with imposed of a "linkage relation" of both arguments.
2.) Indeed, this transformation is such that it does not change the partition function.
So, if we suppose in the partition function
for
spin-system the pull-back relation
due to Eq. (
63) it follows that
where
is a normalization factor. The normalization factor
will not be further considered because it cancels in all thermodynamic averages.
The above equation in conjunction with Eq. (
63) suggested that one can find new couplings
and
(as a function of
K and
h, ). In other words the recursive RG transformation for the 1D Ising could takes place, by demanding
which is nothing but of one-dimensional quadratic maps.
Since the quadratic map
also referred to as the logistic map, appears frequently in the literature we write down the corresponding transformation between both forms:
A curious fact is that if require the condition Eq. (
68) which is coupling with decimation this make an other conjugated form of quadratic map, e.g. Eq. (
69) (as stated first in [
29]) relevant to the problem.
So we obtain exactly the result of [
29] in a different presentation as we will show below. In Ref. [
31] shows that the logistic map
on
is linearly conjugate to the map
on
. Since both maps are "topologically conjugate" i.e. equivalent as far as their dynamics are concerned, one can use any of them to study the other. In particular conjugacy preserves chaos, see. [
31].
Now we will give a more friendly (without using special terminology) deduction of the above statement. Eq.(
68) in explicit form is
It may be rewritten in the form
where
is known as a renormalisation transformation invariant, i.e.
, but here the last property is not essential to our consideration. It can be checked that for
, follows
, and Eq.(
72) imply the well known RG-recursion relation for
K, see e.g. [Kardar Eq.(6.38) p.106]
Finally introducing the new variables [
29]
instead of
in Eq.(
72), one gets
which is exactly the logistic map defined:
The condition
is equivalent to
Thus lhs gets
These constraints define the external magnetic field to lie at the imaginary axis of
h, i.e. on the unit circle of
, with
This makes a connection to Yang-Lee circle theorem, since it is valid also if
is an imaginary field.
Here a small comment is in place. If we introduce
in Eq.(
76) we get
we have the logistic map in the other form
Both expressions appear equal frequently in the literature, see 1.) in Ref. [
32] and 2.) Ref. [
33]. Indeed, a linear transformation
and
brings one to the other. A point
is said to be a fixed point of the map
if
. So, Eq. (
76) has two fixed point
and
Eq. (
82) has two fixed points
and
.
We have thus seen that exploiting a decimation-type renormalization transformation based on the Chebyshev polynomials, Eq. (
63), gives rise to a recursion relation that in turn be mapped on to the logistic equation. This result is not new (see Doland Dolan and Johnston), but it is obtained in a way that reveals the potential of Chebychev recursion relations.
This will exhibit chaos for almost all
, i.e.
. The first inequality implies
h to be imaginary, i.e.
(see eq. (
4)). The second one implies
, see [
29,
34].