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Volume preserving diffeomorphisms with orbital shadowing
 Keonhee Lee^{1} and
 Manseob Lee^{2}Email author
https://doi.org/10.1186/1029242X201318
© Lee and Lee; licensee Springer 2013
Received: 12 July 2012
Accepted: 26 December 2012
Published: 14 January 2013
Abstract
Let f be a volumepreserving diffeomorphism of a closed ${C}^{\mathrm{\infty}}$ Riemannian manifold M. In this paper, we show that the following are equivalent:
(a) f belongs to the ${C}^{1}$interior of the set of volumepreserving diffeomorphisms with orbital shadowing,
(b) f is Anosov.
MSC:37C50, 34D10.
Keywords
1 Introduction
A fundamental problem in differentiable dynamical systems is to understand how a robust dynamic property on the underlying manifold would influence the behavior of the tangent map on the tangent bundle. For instance, in [1], Mañé proved that any ${C}^{1}$ structurally stable diffeomorphism is an Axiom A diffeomorphism. And in [2], Palis extended this result to Ωstable diffeomorphisms. Let M be a closed ${C}^{\mathrm{\infty}}$ Riemannian manifold endowed with a volume form ω. Let μ denote the Lebesgue measure associated to ω, and let d denote the metric induced on M by the Riemannian structure. Denote by ${Diff}_{\mu}(M)$ the set of diffeomorphisms which preserves the Lebesgue measure μ endowed with the ${C}^{1}$topology. We know that every volume preserving diffeomorphism satisfying Axiom A is Anosov (for more details, see [3]).
where ${B}_{\u03f5}(A)$ denotes the ϵneighborhood of a set $A\subset M$. f is said to have the weak shadowing property on Λ (or Λ is weakly shadowable) if for any $\u03f5>0$, there exists $\delta >0$ such that for any δpseudoorbit $\xi ={\{{x}_{i}\}}_{i\in \mathbb{Z}}\subset \mathrm{\Lambda}$, there is a point $y\in M$ such that $\xi \subset {B}_{\u03f5}({\mathcal{O}}_{f}(y))$. Note that if f has the shadowing property, then f has the orbital shadowing property, but the converse is not true (see [7]). It is easy to see that f has the orbital shadowing property on Λ if and only if ${f}^{n}$ has the orbital shadowing property on Λ for $n\in \mathbb{Z}\setminus \{0\}$.
for all $x\in \mathrm{\Lambda}$ and $n\ge 0$.
We denote by ${\mathcal{F}}_{\mu}(M)$ the set of diffeomorphisms $f\in {Diff}_{\mu}(M)$ which have a ${C}^{1}$neighborhood $\mathcal{U}(f)\subset {Diff}_{\mu}(M)$ such that for any $g\in \mathcal{U}(f)$, every periodic point of g is hyperbolic.
Very recently, Arbieto and Catalan [3] proved that every volume preserving diffeomorphism in ${\mathcal{F}}_{\mu}(M)$ is Anosov. To prove this, they used Mañé’s results in [[1], Proposition II.1] and showed that $\overline{P(f)}$ is hyperbolic if $f\in {\mathcal{F}}_{\mu}(M)$. Thus, we have the following theorem.
Theorem 1.1 [[3], Theorem 1.1]
Every diffeomorphism in ${\mathcal{F}}_{\mu}(M)$ is Anosov.
Let $int{\mathcal{OS}}_{\mu}(M)$ denote the ${C}^{1}$interior of the set of volume preserving diffeomorphisms in ${Diff}_{\mu}(M)$ satisfying the orbital shadowing property. In [7], the authors proved that the ${C}^{1}$interior of the set of diffeomorphisms having the orbital shadowing property coincides with the set of structurally stable diffeomorphisms. Note that if a diffeomorphism satisfies structurally stable then it is not Anosov in general. But the converse is true. Finally, we prove the following theorem.
Theorem 1.2 The set ${\mathcal{AN}}_{\mu}(M)$ of Anosov diffeomorphisms in ${Diff}_{\mu}(M)$ coincides with the ${C}^{1}$interior of the set of diffeomorphisms in ${Diff}_{\mu}(M)$ with orbital shadowing; that is, ${\mathcal{AN}}_{\mu}(M)=int{\mathcal{OS}}_{\mu}(M)$.
2 Proof of Theorem 1.2
Remark 2.1 Let $f\in {Diff}_{\mu}^{1}(M)$. From Moser’s theorem (see [8]), we can find a smooth conservative change of coordinates ${\phi}_{x}:U(x)\to {T}_{x}M$ such that ${\phi}_{x}(x)=0$, where $U(x)$ is a small neighborhood of $x\in M$.
Notice that in this definition, only δpseudo orbits of f are contained in Λ, but the shadowing point $y\in M$ is not necessarily contained in Λ. To prove our result, we use Franks’ lemma which is proved in [[9], Proposition 7.4].
Lemma 2.2 Let $f\in {Diff}_{\mu}^{1}(M)$, and $\mathcal{U}$ be a ${C}^{1}$neighborhood of f in ${Diff}_{\mu}^{1}(M)$. Then there exist a ${C}^{1}$neighborhood ${\mathcal{U}}_{0}\subset \mathcal{U}$ of f and $\u03f5>0$ such that if $g\in {\mathcal{U}}_{0}$, any finite finvariant set $E=\{{x}_{1},\dots ,{x}_{m}\}$, any neighborhood U of E, and any volumepreserving linear maps ${L}_{j}:{T}_{{x}_{j}}M\to {T}_{g({x}_{j})}M$ with $\parallel {L}_{j}{D}_{{x}_{j}}g\parallel \le \u03f5$ for all $j=1,\dots ,m$, there is a conservative diffeomorphism ${g}_{1}\in \mathcal{U}$ coinciding with f on E and out of U, and ${D}_{{x}_{j}}{g}_{1}={L}_{j}$ for all $j=1,\dots ,m$.
We introduce the notion of normally hyperbolic which was founded in [10]. Let $V\subset M$ be an invariant submanifold of $f\in {Diff}_{\mu}(M)$. We say that V is normally hyperbolic if there is a splitting ${T}_{V}M=TV\oplus {N}^{s}\oplus {N}^{u}$ such that

the splitting depends continuously on $x\in V$,

${D}_{x}f({N}_{x}^{\sigma})={N}_{f(x)}^{\sigma}$ ($\sigma =s,u$) for all $x\in V$,

there are constants $C>0$ and $0<\lambda <1$ such that for every unit vector $x\in {T}_{x}V$, ${v}^{s}\in {N}_{x}^{s}$ and ${v}^{u}\in {N}_{x}^{u}$ ($x\in V$), we have$\parallel {D}_{x}{f}^{n}\left({v}^{s}\right)\parallel \le C{\lambda}^{n}\parallel {D}_{x}{f}^{n}(v)\parallel \phantom{\rule{1em}{0ex}}\text{and}\phantom{\rule{1em}{0ex}}\parallel {D}_{x}{f}^{n}\left({v}^{u}\right)\parallel \ge {C}^{1}{\lambda}^{1}\parallel {D}_{x}{f}^{n}(v)\parallel $
for all $n\ge 0$.
Proposition 2.3 If $f\in int{\mathcal{OS}}_{\mu}(M)$, then every periodic point of f is hyperbolic.
Proof Take $f\in int{\mathcal{OS}}_{\mu}(M)$, and $\mathcal{U}(f)$ is a ${C}^{1}$neighborhood of $f\in {Diff}_{\mu}(M)$. Let $\u03f5>0$ and $\mathcal{V}(f)\subset {\mathcal{U}}_{0}(f)$ be the number and ${C}^{1}$neighborhood of f corresponding to $\mathcal{U}(f)$ given by Lemma 2.2. To derive a contradiction, we assume that there exists a nonhyperbolic periodic point $p\in P(g)$ for some $g\in \mathcal{V}(f)$. To simplify the notation in the proof, we may assume that $g(p)=p$. Then there is at least one eigenvalue λ of ${D}_{p}g$ such that $\lambda =1$.
Then ${g}_{1}(p)=g(p)=p$.
 (a)
$d({x}_{i},{x}_{i+1})<\delta $ for $i=0,1,\dots ,k1$,
 (b)
$d({x}_{i1},{x}_{i})<\delta $ for $i=0,\dots ,k1$,
 (c)
${x}_{0}=x$ and $d({x}_{k},{x}_{k})>2\u03f5$.
Then ${g}_{1}({\phi}_{p}^{1}(v))={\phi}_{p}^{1}(v)$. Thus, ${g}_{1}^{l}({\mathcal{J}}_{p})={\mathcal{J}}_{p}$ for some $l>0$. Now, we show that if ${\mathcal{J}}_{p}$ is normally hyperbolic for ${g}_{1}$, then the shadowing points belong to ${\mathcal{J}}_{p}$. Assume that there is a shadowing point $y\in M\setminus {\mathcal{J}}_{p}$. Then by the hyperbolicity, there are $l,k\in \mathbb{Z}$ such that $d({g}_{1}^{l}(y),{x}_{k})>\u03f5$, where ${x}_{k}\in \xi ={\{{x}_{i}\}}_{i\in \mathbb{Z}}$. This is a contradiction since ${g}_{1}{}_{{\mathcal{J}}_{p}}$ has the orbital shadowing property. Thus, if ${\mathcal{J}}_{p}$ is normally hyperbolic for ${g}_{1}$, then the shadowing point belongs to ${\mathcal{J}}_{p}$. Since ${g}_{1}{}_{{\mathcal{J}}_{p}}$ has the orbital shadowing property, from the above facts, we have $y\in {\mathcal{J}}_{p}$. But ${g}_{1}^{l}({\mathcal{J}}_{p})={\mathcal{J}}_{p}$ and so ${g}_{1}^{l}{}_{{\mathcal{J}}_{p}}$ is the identity map. Then ${g}_{1}^{l}{}_{{\mathcal{J}}_{p}}$ does not have the orbital shadowing property. Thus, ${g}_{1}{}_{{\mathcal{J}}_{p}}$ also does not have the orbital shadowing property.
With a ${C}^{1}$small modification of the map ${D}_{p}g$, we may suppose that there is $l>0$ (the minimum number) such that ${D}_{p}{g}^{l}(v)=v$ for any $v\in {\phi}_{p}({B}_{\alpha}(p))\subset {T}_{p}M$. Then we can go on with the previous argument in order to reach the same contradiction. Thus, every periodic point of $f\in int{\mathcal{OS}}_{\mu}(M)$ is hyperbolic. □
End of the proof of Theorem 1.2 Let $f\in int{\mathcal{OS}}_{\mu}(M)$. By Proposition 2.3, we see that $f\in {\mathcal{F}}_{\mu}(M)$. Thus, by Theorem 1.1, f is Anosov. □
Declarations
Acknowledgements
We wish to thank the referee for carefully reading of the manuscript and providing us many good suggestions. KL is supported by the National Research Foundation (NRF) of Korea funded by the Korean Government (No. 20110015193). ML is supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology, Korea (No. 20110007649).
Authors’ Affiliations
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