## A non-clustering property of stationary sequences.(English)Zbl 0536.60044

Summary: For a random sequence of events, with indicator variables $$X_ i$$, the behavior of the expectation $$E\{(X_ k+...+X_{k+m-1})/(X_ 1+...+X_ n)\}$$ for $$1\leq k\leq k+m-1\leq n$$ can be taken as a measure of clustering of the events. When the measure on the X’s is i.i.d., or even exchangeable, a symmetry argument shows that the expectation can be no more than m/n. When the X’s are constrained only to be a stationary sequence, the bound deteriorates, and depends on k as well. When m/n is small, the bound is roughly 2m/n for k near n/2 and is like (m/n) log n for k near 1 or n. The proof given is partly constructive, so these bounds are nearly achieved, even though there is room for improvement for other values of k.

### MSC:

 60G10 Stationary stochastic processes 26D15 Inequalities for sums, series and integrals

### Keywords:

cyclic sums; exchangeable; stationary sequence
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