Monge's transport problem on a Riemannian manifold

Monge's problem refers to the classical problem of optimally transporting mass: given Borel probability measures μ + ¬= μ - , find the measurepreserving map s: M → M between them which minimizes the average distance transported. Set on a complete, connected, Riemannian manifold M - and assuming absolute continuity of μ + - an optimal map will be shown to exist. Aspects of its uniqueness are also established.