We present shrinking targets results for general systems with the emphasis on
applications for IETs (interval exchange transformations) $(J,T)$, $J=[0,1)$.
In particular, we prove that if an IET $(J,T)$ is ergodic (relative to the
Lebesgue measure $\lam$), then the equality \[ \liminf_{n\to\infty}\limits n
|T^n(x)-y|=0 \tag{A1} \] holds for $\lam\ttimes\lam$-a. a. $(x,y)\in J^2$. The
ergodicity assumption is essential: the result does not extend to all minimal
IETs. The factor $n$ in (A1) is optimal (e. g., it cannot be replaced by $n
\ln(\ln(\ln n))$.
A sequence $\{x_{n}\}_1^\infty$ in $[0,1)$ is called Borel-Cantelli (BC) if
for all non-increasing sequences of positive real numbers $\{a_n\}$ with
$\underset{i=1}{\overset{\infty}{\sum}}a_i=\infty$ the set
\[\underset{k=1}{\overset{\infty}{\cap}} \underset{n=k}{\overset{\infty}{\cup}}
B(x_n, a_n))=\{x\in[0,1)\mid |x_n-x|<a_n \text{for} \infty
\text{many}n\geq1\}\] has full Lebesgue measure. (To put it informally, BC
sequences are sequences for which a natural converse to the Borel-Cantelli
Theorem holds).
This paper uses a construction of M. Keane to show that there exists a
topologically mixing interval exchange transformation.
This paper proves shrinking target results for IETs. Let {a_1\geq a_2
\geq...} be a sequence of positive real numbers with divergent sum. Then for
almost every IET T, the limsup of B(T^ix,a_i) has full Lebesgue measure (where
B(z, e) is the open ball around z of radius e). Related results are established
including the analogous result for geodesic flows on a translation surface.
Given an irrational alpha and an x in the unit interval, the set of balanced
times, for which the same number of (k*alpha+x) (modulo one) are less than or
equal to one half as are larger than one half, is in general infinite, but
sparse in terms of density. We investigate the sparseness of this sequence in
terms of summation over reciprocals. Our results are that for the generic pair
(alpha,x), the resulting sum diverges, but there are certain exceptional alpha
for which the associated sums converge for every x.