A Class of Potentials for Hyperbolic Transcendental Entire Maps ()
1. Introduction
The study of the thermodynamic formalism of transcendental entire maps has received considerable attention. Notably, the ergodic theory of the exponential family
has been thoroughly examined for a wide range of parameters, as referenced in [1] - [10] and references therein.
When dealing with transcendental entire functions, several challenges arise in the exploration of the thermodynamic formalism. For instance, the Julia set is never compact, a contrast to the situation with polynomials and rational maps. This discrepancy leads to convergence issues, and standard arguments such as the Schauder-Tychonoff Fixed Point Theorem cannot be applied.
Due to the wide-range nature of the transcendental maps, it becomes essential to narrow the scope and consider suitable sub-classes when exploring thermodynamic formalism. In this context, Mayer and Urbański made significant contributions to their paper [10] by providing a complete understanding of the thermodynamic formalism for a large class of hyperbolic meromorphic functions of finite order satisfying a rapid growth condition, associated with a class of tame potentials. Furthermore, recently the authors in [11] provided an overview of the thermodynamic formalism for transcendental meromorphic and entire functions and their geometric applications. In addition, in [12] , they developed an optimal approach to thermodynamic formalism for a wide range of transcendental entire functions whose set of singularities is bounded. Another significant advancement in the field is the development of thermodynamic formalism for random transcendental dynamics. This approach was successfully detailed in [13] .
In the present work, we highlight a class of transcendental entire maps, which includes the exponential family, as defined in Section 2. We show that within this class exist hyperbolic transcendental entire maps that generate a large class of potentials for which the thermodynamic formalism can be effectively applied. The key novelty of our study lies in identifying a class of potentials that deviates from those earlier studied in [10] . We find that the techniques and methods from their work can be adapted with minor modifications to our situation, taking advantage of the properties of the symbolic representation of these maps acting on invariant subsets of Julia sets. These code spaces maintain a natural topology that is inherited from the Euclidean topology.
The paper is organized as follows: In Section 2 we define a class of hyperbolic transcendental entire maps and a class of potentials to state Theorem 1. After gathering several dynamic properties in Theorem 2 and properties of potentials in Proposition 1, the proof of Corollary 1 holds.
2. Hyperbolic Transcendental Entire Maps, Potentials and Results
Given a transcendental entire function
, the Fatou set
is the subset of
where the iterates
of
form a normal family, and its complement is namely called the Julia set, which is denoted by
.
Denote by
the set of finite singularities of the inverse function
, which is the set of critical values (images of critical points) and asymptotic values of
together with their finite limit points. The post-singular set
of
is defined as,
and
is namely called the order of
.
Let
denote the class of transcendental entire functions
satisfying the following properties
1) It is of finite order;
2) Satisfies the rapid derivative growth condition: There are
and
such that for every
we have
;
3) It is of disjoint type, that is, the set
is contained in a compact subset of the immediate basin
of an attracting fixed point
. This is a strong form of hyperbolicity, which was explicitly studied in [14] for instance.
Note that each
belongs to the Eremenko-Lyubich class
It was proved in [15] that for
all the Fatou components of
are simply connected. Hence the immediate basin B is simply connected. Moreover each
is hyperbolic in the sense that the closure of
is disjoint from the Julia set and
is compact. We have that
has no wandering and Baker domains, so B is the only Fatou component of
, see [15] [16] [17] .
Examples in the class
include the family
for
, the family of maps
for
, and
, where
and g is an arbitrary map of finite order such that
is bounded and
is enough small, other examples are the expanding entire maps
,
,
, studied early in [9] .
2.1. Potentials
Fix
. Since the immediate attraction basin
of an attracting fixed point
is simply connected, there exists a bounded simply connected domain
, such that its closure
and boundary
is an analytic Jordan curve. Moreover,
and
, for more details see ( [8] , Lemma 3.1). Following [8] , the pre-images of
by
consists of countably many unbounded connected components called tracts of
. We denote the collection of all these tracts by
.
Since the closure of each tract is simply connected, there exists an open simple arc
, which is disjoint from the union of the closures of all tracts and such that
tends to a point of
as t tends to
, and
tends to
as t tends to
. We use this curve to define the fundamental domains on each tract as follows: since for every
the map
is a cover of
, we have
is the union of infinitely many disjoint simply connected domains S such that the function
is bijective. Given
, we denote by
the collection of connected components of
. The elements of
(1)
are called fundamental domains.
For each
, we have that the restriction
is univalent, so we denote its inverse branch by
. For
and each
denote by
an element of
and put
. Then,
(2)
For each sequence
, let
. Then, the Julia set of
is given by the disjoint union of
, that is
Since
has finite order and of disjoint-type, following [18] , the Julia set
is a Cantor bouquet, that is a union of uncountably many pairwise disjoint curves tending to infinity (hair) and each curve is attached to the unique point accessible from the immediate basin B, called the endpoint of the hair. More precisely, either
is empty or there is a homeomorphism
such that
, and such that for every
we have
. In the latter case
is the only point of
accessible1 from the immediate basin B. See also [7] , which generalizes previous results for the exponential map having an attracting fixed point of [19] .
Following as in [10] , let us consider
be the order of
and
be the corresponding constants of the rapid derivative growth condition of
. Fix
and let
defined by
. Let
be the Riemaniann metric on
defined by
and we derive
with respect to
instead of the Euclidean metric. So, for each
we have
(3)
We consider
, the set of functions
from
to
that are bounded from above and are constant on each element of
. That is, we write
(4)
We define the following class of potentials for
:
Observe that this class contains potentials
, which from (3) are cohomologous to
.
For each
we denote by
the class of tame potentials defined in [10] , that is
Note that the class
does not include most of the functions considered in [10] . However, the class of potentials
determined from
intersects the class of tame potentials, in which the difference is non-empty. So our first result is the following
Theorem 1. There exists
such that
and
.
2.2. Symbolic Representation
Let
be the full shift space, and the shift metric is defined as follows, for some
,
(5)
For every
, we denote a finite word
in
simply by
, so we follows the following notation for cylinders
and for
, we simply denote
. Let
be the left-sided shift map, given by
.
Observe that by definition the set
given in (1) is countably infinite, so we identify
with
. Put
(6)
Let
From (2), we have for each
,
, then the function
on the Julia set
is semi-conjugate to
on X, however
on the set
is conjugate to
. Hence the set X is completely
-invariant.
The set
defined above, is the set of endpoints of hairs
and it satisfies the following properties, it is the set of accessible points from the immediate attraction basin B. It is totally disconnected, however
is connected, see [14] . Moreover, following [20] , the Hausdorff dimension of this set is equal to two, generalizing previous results of Karpińska [3] for the exponential map
with parameters
. This exponential map is probably the best-known example in the family
, its Julia set is a Cantor bouquet and the set of endpoints is modeled by the symbolic space of all allowable sequences, see [19] and [21] .
In the following, we state some properties concerning the dynamics
, endowed with a metric inherited from the euclidean metric on
. It does not necessarily generate the topology induced by the cylinder sets.
Let
, and
defined by
, we have that H induces a metric
on X,
The shift map
is continuous with respect to
.
Given
and
let us write
. For a set
write
. For
and
we define the following sets with respect to the metric
.
For every
and
define
The set X endowed with the metric
is non-compact, however, it can be approximated by an increasing sequence of compact and invariant subsets. Indeed, for all
, define
so, the following holds
Theorem 2.
1) For all
,
,
is compact with respect to
and invariant by
. Moreover, for each compact subset
of X with respect to the metric
, so that
, we have, there exists
, such that
.
2) There exists
such that the following condition holds:
There exist
and
such that for every
and
and
, if
then we have
3) For every
there exists
such that for every
, we have
. Thus
is topologically mixing.
4) The set
is dense in X.
We recall what a conformal measure means; consider a measurable endomorphism
on a measurable space
and a measurable non-negative function g on Y. A measure m on
is called g-conformal for T on g if for all measurable set A which
is measurable and
is invertible we have
(7)
Observe that (7) implies that
is absolutely continuous with respect to m on the σ-algebra
, for every set
such that
is a measurable isomorphism.
Corollary 1. Let
. Then for every potential
we have the following properties.
1) The topological pressure
. exists and is independent of
.
2) There exists a unique
-conformal measure
of
.
3) There exists a unique probability Gibbs state
. That is,
is f-invariant and equivalent to
. Moreover, both measures are ergodic and supported on the radial Julia set
, where
4) The density
is a nowhere vanishing continuous and bounded function on the Julia set
.
3. Proof of Results
3.1. Proof of Theorem 1
Consider the exponential family
. Each
belongs to
, because it has order equal to 1, satisfies the rapid derivative growth condition with
and
, and since 0 is the only singular value of
. Thus this map is hyperbolic. Moreover the potentials
, where
, are tame potentials and also belong to the class
.
On the other hand, let
be the open unit disk in
, then
and since
we have
. Moreover, since the immediate basin B of the attracting fixed point is the only Fatou component of
we have
.
Since
, the only tract of
is the half plane
. Let us consider the ray
defined by
, then
and for each
, put
. Then
is the disjoint union of the fundamental domains
.
Following [6] , let
be a function such that for each
, this function is constant on
and we denote by
its value on this set. Furthermore we assume that the sequence
of positive numbers satisfies
(8)
Define
, where
,
if
and the sequence
satisfies (8). Observe that any potential as above
satisfies
, so,
is not a tame potential, however, this belongs to the class
since the function c is bounded on each
and
.
3.2. Proof of Theorem 2
1) The classical Denjoy-Carleman-Ahlfors Theorem [22] implies that transcendental entire functions of finite order have only a finite number of tracts. We will assume for simplicity that for
there is only one tract T, and there is no complication in the generalization to a finite number of tracts.
Let
and denote by
the union of
fundamental domains in T, that is
and define
Let
be a sequence in
. Taking a subsequence if it is necessary, one can assume that for some R large enough, the subsequence
is contained in
. So for every
there is there is an endpoint
. Since
is bounded and
is closed in
we have there is a subsequence converging to some point
. Let
be the itinerary associated to z, then
and
,
.
On the other hand, let
be a compact subset of X with respect to the metric
with
. Let
and let
with itinerary
, then since the compact subset
intersects only a finite numbers of tracts (see ( [8] , Lemma 3.2)), there exists
such that for every
we have
. Therefore
.
2) Follows from the derivative grown condition and the uniformly expanding property of
, see ( [10] , Proposition 4.4).
3) This a standard fact described in ( [10] , Lemma 4.2). However, we include a short proof. That is, for each
there exists
such that for every
,
, then the property follows. Let
and
,
, for
. By expanding property in Part 2, follows there is
such
. Let R be large enough such that
and there is
such that
So,
. Taking
, we have for every
,
.
4) This property can be inferred from the general property of the density of periodic sources in the Julia set, as referred in ( [23] , Theorem 4). However, we include here a short proof: Let
and
. Then, there exists
such that
It is enough to take
such that
. So, we have
Hence,
. Since
, then for some
we have
. Moreover from Part 3, there is
such that
. Therefore, for
we follow that
. Hence, the set
contains the sequence
. Let
such that
, then
. Then, taking
we conclude
.
3.3. Proof of Corollary 1
Let
be denote the Banach space of bounded continuous functions on
. For each potential
, the transfer operator associated to
and denoted by
acts continuously on
. So for each
,
To prove Corollary 1 one can adapt with minor modifications the approach given in [10] on the thermodynamic formalism for a large class of hyperbolic meromorphic functions
of finite order
satisfying a rapid growth condition and for a class of tame potentials, to the family of transcendental entire maps and potentials under study. Therefore, following as in [10] , we have that the following proposition remains valid for potentials
with c being only bounded from above.
Proposition 1. Given
, we have each potential
satisfies the following properties.
1)
2)
3)
Proof. Let
be a potential in
and
,
Since
satisfies the derivative growth condition, we have
Since
is a transcendental entire function of finite order
and
, then the Borel-Picard Theorem (see ( [10] , Theorem 3.5)) states that the series has the exponent of convergence equal to
. So the last sum is finite. Following ( [10] , Proposition 3.6), there exists
such for all
we have
(9)
So, the Equation (9) implies
and
. □
Acknowledgements
The author was partially supported by Mathamsud Project TOMCAT 22-MATH-10.
NOTES
1If U is simply connected domain in the Riemann sphere
, we say that a point
is accessible from U if there exists a curve
such that
.