Let $({\mathcal X},d,��)$ be a metric measure space satisfying both the upper doubling and the geometrically doubling conditions. In this paper, the authors establish some equivalent characterizations for the boundedness of fractional integrals over $({\mathcal X},d,��)$. The authors also prove that multilinear commutators of fractional integrals with ${\mathop\mathrm{\,RBMO(��)}}$ functions are bounded on Orlicz spaces over $({\mathcal X},d,��)$, which include Lebesgue spaces as special cases. The weak type endpoint estimates for multilinear commutators of fractional integrals with functions in the Orlicz-type space ${\mathrm{Osc}_{\exp L^r}(��)}$, where $r\in [1,\infty)$, are also presented. Finally, all these results are applied to a specific example of fractional integrals over non-homogeneous metric measure spaces.

Taiwanese J. Math. (to appear)