Krengel-Lin decomposition for probability measures on hypergroups

Abstract

A Markov operator $P$ on a $\sigma$-finite measure space $(X, \Sigma, m)$ with invariant measure $m$ is said to have Krengel-Lin decomposition if $L^2 (X) = E_0 \oplus L^2 (X,\Sigma_d)$ where $E_0 = \{f \in L^2 (X) \mid ||P^n (f) || \ra 0 \}$ and $\Sigma_d$ is the deterministic $\sigma $-field of $P$. We consider convolution operators and we show that a measure $\lam$ on a hypergroup has Krengel-Lin decomposition if and only if the sequence $(\check \lam ^n *\lam ^n)$ converges to an idempotent or $\lam$ is scattered. We verify this condition for probabilities on Tortrat groups, on commutative hypergroups and on central hypergroups. We give a counter-example to show that the decomposition is not true for measures on discrete hypergroups which is in contrast to the discrete groups case.