## Tag Archives: Bowen

### Asymmetry of Bowen’s dimensional entropy

1. Bowen and Dinaburg gave a alternative definition of topological entropy $h_{\text{top}}(f)$ by calculating the exponential growth rate of the $(n,\epsilon)$-covers. This definition resembles the box dimension of Euclidean subset $E\subset\mathbb{R}^k$, and gives the same value while using the definition given by Adler, Konheim, and McAndrew. In particular, the entropy is time-reversal invariant: $h_{\text{top}}(f^{-1})=h_{\text{top}}(f)$.

2. Later Bowen introduced another definition of topological entropy for noncompact subset in 1973, which resembles the Hausdorff dimension.
Let $f:X\to X$ be a homeomorphism on a compact metric space, $E\subset X$ and $h_B(f,E)$ be Bowen’s topological entropy of $E$ (may not be compact).

Bowen proved that, for any ergodic measure $\mu$, $h_B(f,G_{\mu})=h(f,\mu)$, where $G_{\mu}$ is the set of $\mu$-generic points. This identity has been generalized to general invariant measures of transitive Anosov systems:

Theorem 1. (Pfister–Sullivan link) Let $f:M\to M$ be a transitive Anosov diffeomorphism. Then $h_B(f,G_{\mu})=h(f,\mu)$ for any invariant measure $\mu$.

Note that $\mu(G_\mu)=0$ whenever $\mu$ is invariant but non-ergodic.

3. An interesting fact is that $h_B(f,E)$ may not be time-reversal invariant.

Example 2. Let $f:M\to M$ be a transitive Anosov diffeomorphism, $p$ be a periodic point, $D=W^u(x,\epsilon)$. Then $h_B(f,D) > 0$, but $h_B(f^{-1},D)=0$.

Now let $\mu,\nu$ be two different invariant measures of $f$, $W^s(\mu,f)=G_\mu$ be the set of $\mu$-generic points with respect to $f$, and $W^u(\nu,f)=W^s(\nu,f^{-1})$ be the set of $\mu$-generic points with respect to $f^{-1}$. Let $H_f(\mu,\nu)=B^s(\mu,f)\cap B^u(\nu,f)$ (resemble the heteroclinic intersection of different saddles). Then it is proved (Proposition D in here) that

Proposition 3. Let $f:M\to M$ be a transitive Anosov diffeomorphism. Then $h_B(f,H_f(\mu,\nu))=h_\mu(f)$ and $h_B(f^{-1},H_f(\mu,\nu))=h_\nu(f)$.

A well known fact is that, for any $0\le t\le h_{\text{top}}(f)$, there exists some invariant measure $\mu$ with $h_\mu(f)=t$. So a direct corollary of Proposition 3 is:

Corollary. Let $f:M\to M$ be a transitive Anosov diffeomorphism. Then for any $a, b\in [0, h_{\text{top}}(f)]$, there exists an invariant subset $E$ such that $h_B(f,E)=a$ and $h_B(f^{-1},E)=b$.

### Generic Anosov does not admit fat horseshoe

To start let’s describe an interesting proposition in QIU Hao’s paper (Commun. Math. Phys. 302 (2011), 345–357.)

Assume $f\in\mathrm{Diff}^1(M)$ and $\Lambda$ be a basic (isolated and transitive, or mixing) hyperbolic set of $f$. It is well known (Anosov) that there exists an open neighborhood $\mathcal{U}\ni f$ and an open set $U\supset\Lambda$ such that for each $g\in\mathcal{U}$,

1). $\Lambda_g=\bigcup_{n\in\mathbb{Z}}g^nU$ is an isolated hyperbolic set of $g$. Moreover $\Lambda_g\to\Lambda$ as $g\to f$.
2). there exists a (Holder) homeomorphism $h_g:\Lambda\to\Lambda_g$ such that $h_g\circ f(x)=g\circ h_g(x)$ for every $x\in\Lambda$. Moreover $h_g\to \mathrm{Id}$ as $g\to f$.

Now let’s consider the unstable log-Jacobian $\phi_g\in C(\Lambda_g,\mathbb{R})$ as
$\phi_g(x)=-\log\det(D_xg:E^u_g(x)\to E^u_g(gx)), x\in\Lambda_g$.

By classical hyperbolic theory (Sinai, Ruelle and Bowen), we know that for each $g\in\mathcal{U}\cap \mathrm{Diff}^2(M)$, the topological pressure $P(\phi_g;g,\Lambda_g)=0$ and there exists a unique equilibrium state of $\phi_g$ with respect to $(g,\Lambda_g)$.

—————————————————————–
Define a map $\Phi:\mathcal{U}\to C(\Lambda,\mathbb{R})$ by $\Phi(g)(x)=\phi_g(h_g(x))$. It takes a few seconds to see that $\Phi$ is continuous.

Proposition 3.1 (Qiu) For each $g\in\mathcal{U}$, $P(\phi_g;g,\Lambda_g)=0$.
Proof. Since topological pressure is invariant under topological conjugation, we have
$P(\phi_g;g,\Lambda_g) = P(\Phi(g);\Lambda,f)$.
Now we pick $g_k\in\mathcal{U}\cap \mathrm{Diff}^2(M)$ with $g_k\to f$. Note this also implies $h_{g_k}\to\mathrm{Id}$, $\Phi(g_k)\to\phi_f$. So
$P(\phi_f;f,\Lambda)=\lim_{k\to\infty}P(\Phi(g_k);f,\Lambda)=\lim_{k\to\infty}P(\phi_{g_k};g_k,\Lambda_{g_k})=0$.
This finishes the proof.

Remark: In particular for all Anosov diffeomorphisms and all Axiom A diffeomorphisms with no cycle condition, we have $P(\phi_f;f)=0$.

Proposition 3.1 (continued). for generic $g\in\mathcal{U}$, there exists a unique equilibrium state for $\phi_g$ with respect to $(g,\Lambda_g)$.
Proof. Since $(f,\Lambda)$ is expansive, the entropy map $\mathcal{M}(f,\Lambda)\to\mathbb{R},\mu\mapsto h(f,\mu)$ is upper semicontinuous and there is a residual subset $\mathcal{R}\subset C(\Lambda,\mathbb{R})$ such that each $\phi\in\mathcal{R}$ has a unique equilibrium state with respect to $(f,\Lambda)$. Since $\Phi$ is continuous, the pre-image $\Phi^{-1}(\mathcal{R})$ is

1. a $G_\delta$ set in $\mathcal{U}$ since the pre-images of open sets are open;

2. a dense set in $\mathcal{U}$ since $\Phi(g)\in\mathcal{R}$ for all $g\in\mathcal{U}\cap\mathrm{Diff}^2(M)$.

In particular $\Phi^{-1}(\mathcal{R})$ is residual in $\mathcal{U}$. The proof is complete.

We focus on a special case of QIU’s main result. Let $\mathcal{A}^r$ be the set of $C^r$ Anosov diffeomorphisms on $M$ (might be empty). For an invariant measure $\mu$, we let $B(\mu)$ be the set of points with $\frac{1}{n}\sum_{0\le k.

Theorem A (Qiu). Generic $f\in\mathcal{A}^1$ has a unique SRB measure $\mu_f$: $m(M\backslash B(\mu_f))=0$.
Indeed, $\mu_f$ is the unique equilibrium state of $\phi_f$ (hence ergodic).

Robinson and Young constructed an Anosov diffeomorphism with nonabsolutely continuous foliations, by embedding an Bowen horseshoe $\Lambda_B$ to some $f\in\mathcal{A}(\mathbb{T}^2)$. Although $f$ is transitive, every point in $\Lambda_B$ can not be a transitive point. Theorem A implies that this phenomenon fails generically:

Observation: generic $f\in\mathcal{A}^1(M)$ does not admit Bowen’s fat horseshoe.
Proof. A priori, we donot know if every Anosov is transitive. So we divide $\mathcal{A}^1(M)=\mathcal{A}_t(M)\sqcup \mathcal{A}_e(M)$ into transitive ones and exotic ones. But we know they are always structurally stable. Therefore both parts are open.

By Theorem A, we know that, for generic $f\in\mathcal{A}_t(M)$, $\mu_f$ is ergodic and fully supported. Hence every point in $B(\mu_f)$ is a transitive point. In particular every closed invariant set of $f$ has trivial volume: 0 or 1.

For maps in the exotic ones $f\in\mathcal{A}_e(M)$, at least they can be viewed as Axiom A system, and Smale’s Spectra Decomposition Theorem applies: $\Omega(f)=\Lambda_1\sqcup\cdots\sqcup\Lambda_n$, where $n=n(f)$ is locally constant. Some of them are attractors, say $A_1,\cdots, A_k$, some are repellers, say $R_1,\cdots, R_l$. Let $\mathcal{R}_0$ be the residual subset given by QIU for all repellers. Clearly $\mathcal{R}=\mathcal{R}_0\cap\mathcal{R}^{-1}_0$ is also generic. For each $f\in \mathcal{R}$, there is an SRB $\mu_{u,i}$ relative to $R_i(f)$ with $m(\bigcup_i B(\mu_{u,i}))=1$ and an SRB $\mu_{s,j}$ relative to $A_{j}$ for $f^{-1}$ with $m((\bigcup_j B(\mu_{s,j}))=1$. Incorrect conclusion. The following are void.

To derive a contradiction, suppose that there was a fat horseshoe $H$ of $f$, then

1. either $H\cap B(\mu_{u,i})=\emptyset$, (contradicts $m(\bigcup_i B(\mu_{u,i}))=1$);

2. or $H\cap B(\mu_{s,j})=\emptyset$, (contradicts $m(\bigcup_j B(\mu_{s,j}))=1$);

3. or there exist $x\in H\cap B(\mu_{u,i})$ and $y\in H\cap B(\mu_{s,j})$. In particular $H$ has nontrivial intersections with the attarctor $A_j$ and the repeller $R_i$ simultaneously, which contradict the transitivity of Horseshoe. Q.E.D.