We study a multi-marginal optimal transportation problem with cost function of the form $c(x_{1}, \ldots,x_{m})=\sum_{k=1}^{m-1}|x_{k}-x_{k+1}|^{2} + |x_{m}- F(x_{1})|^{2}$, where $F: \mathbb{R}^...

This note describes some recent results on the regularity of optimal transport maps. As we shall see, in general optimal maps are not globally smooth, but they are so outside a “singular set” of measure zero. 1. The optimal transportation problem The optimal transportation problem, whose origin dates back to Monge [19], aims to find a way to transport a distribution of mass from one place to an...