ISRAEL JOURNAL OF MATHEMATICS, cilt.136, ss.373-379, 2003 (SCI-Expanded)
We prove that a, Markov operator T on L-1 has an invariant density if and only if there exists a density f that satisfies lim sup(n-->infinity) parallel toT(n) f-fparallel to < 2. Using this result, we show that a Frobenius-Perron operator P is mean ergodic if and only if there exists a density w such that lim sup(n-->infinity) parallel toP(n)f - wparallel to < 2 for every density f. Corresponding results hold for strongly continuous semigroups.