The fractional maximal operator and fractional integrals on variable L^p spaces

We prove that if the exponent function p((.)) satisfies log-Holder continuity conditions locally and at infinity, then the fractional maximal operator M(alpha), 0 < alpha < n, maps L(p(.)) to L(q(.)), where 1/p(x) - 1/q(x) = alpha/n. We also prove a weak-type inequality corresponding to the weak (1, n/(n - a)) inequality for M(alpha). We build upon earlier work on the Hardy-Littlewood maximal operator by Cruz-Uribe, Fiorenza and Neugebauer [3]. As a consequence of these results for M(alpha), we show that the fractional integral operator I(alpha) satisfies the same norm inequalities. These in turn yield a generalization of the Sobolev embedding theorem to variable L(p) spaces.