American Institute of Mathematical Sciences

$ \lim_{n\to\infty}\frac{\log K_n(x)}{n}=1 \quad$and$\quad \lim_{n\to\infty}\frac{\log K_n(x,y)}{n}=1 \quad $a.e.

$ \tau_{B(x,r)}(x)=$inf{$1\le k \le n | S^k x \in B(x,r)},$

of a point $x$ to the ball $B(x,r)$ as $r$ tends to $0$. For a function $\varphi:(0,\infty)\to\mathbb R$, let A$(\varphi(r))$ denote the set of accumulation points of $\varphi(r)$ as $r\to 0$. We show that the first return time exponent, $\frac{\log\tau_{B(x,r)}(x)} {-\log r}$, has an extremely complicated and surprisingly intricate structure: for any compact subinterval $I$ of $(0,\infty)$, the set of points $x$ such that for each $t\in I$ there exists arbitrarily small $r>0$ for which the first return time $\tau_{B(x,r)}(x)$ of $x$ to the neighborhood $B(x,r)$ behaves like $1/r^t$, has full Hausdorff dimension on any open set, i. e.

dim$(G\cap ${$x\in K| $A ($\frac {\log\tau_{B(x,r)}(x)}{-\log r}) =I$}) $=$dim $K$

for any open set $G$ with $G\cap K$≠$\emptyset$. As a consequence we deduce that the so-called multifractal formalism fails comprehensively for the first return time multifractal spectrum. Another application of our results concerns the construction of a certain class of Darboux functions.

$ \ddot{q} = \alpha(\omega t) V'(q), \quad t \in \mathbb R, q \in \mathbb R^N,$ $\qquad\qquad (L_\omega)$

has, for some classes of functions $\alpha$, a chaotic behavior---more precisely the system has multi-bump solutions---for all $\omega$ large. These classes of functions include some quasi-periodic and some limit-periodic ones, but not any periodic function.
We prove the result using global variational methods.