# Eventually Periodic Points of Infra-Nil Endomorphisms

- KuYong Ha
^{1}Email author, - HyunJung Kim
^{1}and - JongBum Lee
^{1}

**2010**:721736

https://doi.org/10.1155/2010/721736

© Ku Yong Ha et al. 2010

**Received: **14 August 2009

**Accepted: **19 February 2010

**Published: **28 February 2010

## Abstract

Hyperbolic toral automorphisms provide important examples of chaotic dynamical systems. Generalizing automorphisms on tori, we study (infra-)nil endomorphisms defined on (infra-)nilmanifolds. In particular, we show that every infra-nil endomorphism has dense eventually periodic points.

## 1. Introduction

Let be an nonsingular integer matrix. Then induces a map on the -torus . If is hyperbolic, we say that is a hyperbolic toral endomorphism. If, in addition, , then is called a hyperbolic toral automorphism.

A hyperbolic toral automorphism provides an important example of a chaotic dynamical system. We review the most fundamental property about hyperbolic toral automorphisms, together with some definitions which are necessary to describe this property. See [1] for details.

A continuous surjection
of a topological space
is said to be *topologically transitive* if, for any pair of nonempty open sets
and
in
, there exists
such that
. Intuitively, a topologically transitive map has points which eventually move under iteration from one arbitrary small neighborhood to any other. The continuous map
of the metric space
is said to have *sensitive dependence on initial conditions* if there exists
such that, for any
and any neighborhood
of
, there exist
and
such that
. Intuitively, a map possesses sensitive dependence on initial conditions if there exist points arbitrarily close to
which eventually separate from
by at least
under iteration of
.

The following proposition shows that a hyperbolic toral automorphism is dynamically quite different from its linear counterpart.

Proposition 1.1 (see [1, Theorem ]).

A hyperbolic toral automorphism is chaotic on . That is,

()the set of periodic points of is dense in ;

() is topologically transitive;

() has sensitive dependence on initial conditions.

Anosov diffeomorphisms play an important role in dynamics. In [2], Smale raised the problem of classifying the closed manifolds (up to homeomorphism) which admit an Anosov diffeomorphism. Franks [3] and Manning [4] proved that every Anosov diffeomorphism on an infra-nilmanifold is topologically conjugate to a hyperbolic infra-nil automorphism. In [5], Gromov proved that every expanding map on a closed manifold is topologically conjugated to an expanding map on an infra-nilmanifold.

We will consider infra-nil *endomorphisms* in this paper. These include Anosov diffeomorphisms and expanding maps on infra-nilmanifolds up to topological conjugacy. The purpose of this paper is to show that the infra-nil endomorphisms have dense eventually periodic points. In the case of infra-nil automorphisms, this is already known (cf. [4, Lemma
]).

## 2. Toral Endomorphisms

Now we show that every toral *endomorphism* has dense periodic points. This generalizes [1, Proposition
] in which it is shown that every toral automorphism has dense periodic points.

Definition 2.1.

For a self-map
, a point
of
is called an *eventually periodic point* of
if
for some
. If
, then it becomes a periodic point of
with period
.

Note that if is a nonempty set of prime numbers, then the set is a multiplicative subset of . Let be the ring of quotients of by . We denote by . Clearly, and .

Lemma 2.2.

Let be a toral endomorphism of the torus induced by the automorphism and let be a nonempty set of prime numbers. Then every point with coordinates in is an eventually periodic point of . Moreover, if for all , then every point with coordinates in is a periodic point of .

Proof.

Let be a point of with coordinates in . Finding a common denominator, we may assume that is of the form where and are integers. Write . Then there are exactly points in of the form with .

Hence . Therefore, is a periodic point of .

Corollary 2.3.

Every toral endomorphism of the torus has dense periodic points.

Proof.

Let be a prime number with and let . Then by Lemma 2.2, the points with coordinates in are periodic. Moreover, , the set of points in with coordinates in , is a dense subset of the torus .

## 3. Nil Endomorphisms

In this section, we first recall from [6–10] some definitions about nilpotent Lie groups and give some basic properties which are necessary for our discussion.

Let
be a connected, simply connected nilpotent Lie group. A discrete cocompact subgroup
of
is said to be a *lattice* of
, and in this case, the quotient space
is said to be a *nilmanifold*.

of
in such a way that
is the group generated by
and
for each
. We refer to
as a *preferred basis* of
.

We use to indicate the Lie algebra of . This Lie algebra has the same dimension and nilpotency class as . Moreover, in the case of connected, simply connected nilpotent Lie groups it is known that the exponential map is a diffeomorphism. We denote its inverse by . If is another connected, simply connected nilpotent Lie group, with Lie algebra , then we have the following properties.

(i)For any homomorphism of Lie groups, there exists a unique homomorphism (differential of ) of Lie algebras, making the following diagram commuting:

(ii)Conversely, for any homomorphism of Lie algebras, there exists a unique homomorphism of Lie groups, making the above diagram commuting.

If
is a preferred basis of
, then
can be regarded as a basis for the vector space
. We call the basis
of
*preferred*. In particular, if
is a lattice of
, then every preferred basis
of
becomes a preferred basis
for the vector space
.

We first generalize the concept of toral automorphisms to that of nil endomorphisms and show that every nil endomorphism has eventually dense periodic points.

Lemma 3.1.

where the diagonal blocks 's are integral matrices, and . In particular, the automorphism on restricts to an automorphism on a lattice of if and only if its differential has determinant ± .

The proof of this lemma is rather straight forward and so we omit the proof. See, for example, [11, Lemma ] and [12, Proposition ].

Definition 3.2.

Let be a nilmanifold and let be an automorphism with . Then induces a surjective map on the nilmanifold , which is one of the following two types.

(I)
has determinant of modulus 1. In this case
is called a *nil automorphism*.

(II)
has determinant of modulus greater than 1. In this case
is called a *nil endomorphism*.

If, in addition,
is hyperbolic (i.e.,
has no eigenvalues of modulus 1), then we say that the nil automorphism or endomorphism
is *hyperbolic*.

Example 3.3.

Thus we see that the differential of any automorphism on has determinant and eigenvalues and . Thus if , then has an eigenvalue of modulus 1. Therefore, there are no hyperbolic nil automorphisms on any nilmanifold . (There are examples of hyperbolic nil, nontoral, automorphisms. In fact, we can find such examples from many literatures. For example, we refer to [2, 14–18].)

Note that the first one has eigenvalues of modulus all greater than , and the second one has determinant of modulus greater than 1, and there is at least one eigenvalue with modulus less than 1.

Corollary 3.4.

If is a nil automorphism, then the automorphism induces a nil automorphism which is . In particular, is a diffeomorphism of .

We refer to this preferred basis
as a *canonical basis* of
. Given
, we use
to denote the element of
whose canonical coordinate is
. Thus, we have an identification
sending
to
.

Among interesting properties of this identification, we recall the following ([7, Theorem 2.1.(3)]): for any homomorphism , there exists a polynomial function with rational coefficients such that for all . Moreover, any homomorphism of extends to a homomorphism of by using the same polynomial.

Example 3.5.

Lemma 3.6.

For any homomorphism , the associated polynomial function has coefficients in .

Proof.

which is of the form (3.22) claimed above.

where is a polynomial with coefficients in . We will use this fact below.

where the are polynomials with coefficients in . Therefore, using (3.26), this implies that the polynomial is as required.

Remark 3.7.

Our original proof was longer treating the case where is a -step nilpotent Lie group. This one was provided by one of the referees.

Now we fix a canonical basis
of
. A point
of the nilmanifold
is said to have *rational coordinates* or simply
has rational coordinates if
for some
. First we show that if
and
with
, then
for some
. We recall the following ([7, Theorem
.(
)]): there exists a polynomial function with rational coefficients
satisfying
for all
. The group product on
is defined using this polynomial
. Now, suppose that
and
with
. Then
for some
. Since
,
for some
. Hence we have
. Since
, and
is a polynomial function with rational coordinates, we must have
. This proves our assertion. Therefore the points of
with rational coordinates are well defined. Consequently for a subring
of
with
, the points of
with coordinates in
are well defined.

It is known that every (infra-)nil automorphism has dense periodic points (see the proof of [4, Lemma ]). Now we will generalize this to the case of (infra-)nil endomorphisms. The proof below is exactly the same as that of Lemma 2.2, except that the coefficients involved are different and hence Lemma 3.6 is essential.

Theorem 3.8.

Let be a nil endomorphism of the nilmanifold . Let be a ring obtained from by adding finitely many primes , that is, . Then every point with coordinates in is an eventually periodic point of . Moreover, if for all , then every point with coordinates in is a periodic point of .

Proof.

We will show this by induction on the nilpotency class of . If , then is a torus and this case is proved in Lemma 2.2.

Now let and assume that the assertion is true for any connected, simply connected nilpotent Lie group of nilpotency class and for any ring obtained from by adding finitely many primes.

The canonical basis of induces the canonical bases and of and , respectively, where stands for the image of in under the natural surjection . Hence the points in with rational coordinates are well defined. Furthermore the points in with rational coordinates are also well-defined.

which implies that is an eventually periodic point of .

Moreover, if for all , then by Lemma 2.2 and induction hypothesis, we can choose and so . Thus is a periodic point of .

Corollary 3.9.

Every nil endomorphism of the nilmanifold has dense eventually periodic points.

Proof.

Using the fact that the points of with coordinates in are dense in , we obtain the result.

Example 3.10.

is not a periodic point, but an eventually periodic point of with least period (i.e., an eventually fixed point). Note here that and is the coefficient coming from the nilpotent Lie group .

At this moment, we donot know whether Corollary 3.9 is true for periodic points in the general case, that is, the case where for some . We now propose naturally the following problem.

Question 1.

Every nil endomorphism has dense periodic points.

Corollary 3.11.

Every nil automorphism of the nilmanifold has dense periodic points.

Proof.

## 4. Infra-Nil Endomorphisms

Let
be a connected, simply connected nilpotent Lie group and let
be a maximal compact subgroup of
. A discrete and cocompact subgroup
of
is called an *almost crystallographic group*. Moreover, if
is torsion-free, then
is called an *almost Bieberbach group* and the quotient space
an *infra-nilmanifold*. In particular, if
, then
is a nilmanifold. Recall from [19] that
is the maximal normal nilpotent subgroup of
with finite quotient group
, called the *holonomy group* of
.

Definition 4.1.

*weakly*

*-equivariant*; that is, there is a homomorphism of such that

Then induces a surjection , which is one of the following types.

(I)
has determinant of modulus 1. In this case
is called an *infra-nil automorphism*.

(II)
has determinant of modulus greater than 1. In this case
is called an *infra-nil endomorphism*.

If, in addition,
is hyperbolic, then we say that the infra-nil automorphism or endomorphism
is *hyperbolic*.

Since for all , we have for all . Hence is the unique extension of the homomorphism of the lattice of . If is an isomorphism, then is also an isomorphism. Conversely, assume that is an isomorphism. Using the fact that is torsion-free, we can show that is injective. This fact implies that is also injective on the finite group and hence must be an isomorphism. Therefore, is an isomorphism. (The converse was suggested by a referee.) If is an infra-nil automorphism, then being implies by Lemma 3.1 that is an isomorphism and thus is a nil automorphism, and vice versa. Note also that is an infra-nil endomorphism if and only if is a nil endomorphism.

Let denote the set of eventually periodic points of a self-map .

Theorem 4.2.

Every infra-nil endomorphism has dense eventually periodic points.

Proof.

## Declarations

### Acknowledgments

The authors would like to thank the referees for pointing out some errors and making careful corrections to a few expressions in the original version of the paper. The authors also would like to thank both referees for suggesting the apt title. The first author was partially supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2005-206-C00004), and the third author was supported in part by KOSEF Grant funded by the Korean Government (MOST) (no. R01-2007-000-10097-0).

## Authors’ Affiliations

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