## Tag Archives: Lyapunov exponent

### nonuniform conditions and uniform conclusions

Let $f:X\to X$ be a continuous map. CAO Yongluo proved that if $\psi\in C(X,\mathbb{R})$ is continous and $\mu(\psi) < \lambda$ for all $\mu\in\mathcal{M}(f)$, then
for EVERY $x\in X$, there exists $n_x\ge 1$ such that the Birkhoff average $\psi_f(x,n_x) < \lambda$;
Moreover there exists a uniform $N\ge1$ such that $\psi_f(x,n) < \lambda$ for every $x\in X$ and $n\ge N$.

Now let $\phi_n: X\to \mathbb{R}$ be a family of continuous subadditive potential with respect to $(X,f)$. By subadditive ergodic theorem we know that the limit $\phi_f(x)=\lim_{n\to\infty} \frac{\phi_n(x)}{n}$ exists on a set of full probability.

CAO: If $\phi_f(x) < 0$ on a set of full probability, then there exists a uniform $L\ge 1$ and $\lambda < 0$ such that
$\int \frac{\phi_N}{N} d\mu < \lambda$ for every $\mu\in \mathcal{M}(f)$.

### Katok’s closing property for Pesin blocks

First let us list some interesting results of Katok (Lyapunov exponents, entropy and periodic orbbits for diffeomorphisms 1980).

Let $f:M\to M$ be a $C^r$ diffeo for some $r>1$.
I. If there exists a nonatomic ergodic hyperbolic measure $\mu$ then $h_{\mathrm{top}}(f)>0$.
II. Generally $\limsup_{n\to\infty}\frac{\log |P_n(f)|}{n}\ge h_\mu(f)$ for each hyperbolic measure $\mu$.

He started with that for a hyperbolic ergodic measure $\mu$, the Pesin set has full $\mu$-measure. Katok first proved that a recurrent point with respect to the Pesin block is closable and the closed orbit is hyperbolic whose invariant manifolds are of uniform size (depending on the level of block).

Then if the measure is continuous, he picked two points $x_i$ in the support of $\mu$ close to each other and then two recurrent points $y_i\in\Lambda_k$ that are arbitrary close to $x_i$ and found a periodic point $p_i$ for each one. The new points are so close to the recurrent point $d(y_i,p_i)\le d(x_1,x_2)/100$ that

1. these two are distinct and
2. they are still close: $d(p_1,p_2)\le 2d(x_1,x_2)$.

Since their invariant manifolds have uniform size (with respect to level of Pesin block) and the $p_i$‘s can be really close, they intersect transversally. This guarantees the existence of horseshoe. So the map has positive topological entropy (although the metric entropy of $\mu$ might be zero).

For the later claim he reduced to the case that $\mu$ is ergodic. Then pick a subset $K_n$ of Pesin blocks with cardinal growth approximating the metric entropy that is $d^n_f$-separated and recurrent (closer than the separate constant) for some time during $[n,(1+\epsilon)n]$. This involves a counting definition of metric entropy given in the first section. Then for each point $x\in K_n$ we find a periodic point $p_x$ closing $x$. Again these periodic points are distinct (and hyperbolic) with period in $[n,(1+\epsilon)n]$. Thus we have
$\sum_{j=n}^{(1+\epsilon)n}|P_j(f)|\ge |K_n|$. So the growth of hyperbolic periodic points dominates the metric entropy of any (ergodic and hence invariant) hyperbolic measure.