On Sudakov's type decomposition of transference plans with norm costs

Abstract

We consider the original strategy proposed by Sudakov for solving the Monge transportation problem with norm cost $|\cdot|_{D^*}$ \[ \min \bigg\{\int |\mathtt T(x) - x|_{D^*} d\mu(x), \ \mathtt T : \mathbb R^d \to \mathbb R^d, \ \nu = \mathtt T_\# \mu \bigg\}, \] with $\mu$, $\nu$ probability measures in $\mathbb R^d$ and $\mu$ absolutely continuous w.r.t. $\mathcal L^d$. The key idea in this approach is to decompose (via disintegration of measures) the Kantorovich optimal transportation problem into a family of transportation problems in $Z_\mathfrak a\times\mathbb R^d$, where $\{Z_\mathfrak a\}_{\mathfrak a\in\mathfrak A} \subset \mathbb R^d$ are disjoint regions such that the construction of an optimal map $\mathtt T_\mathfrak a : Z_\mathfrak a \to \mathbb R^d$ is simpler than in the original problem, and then to obtain $\mathtt T$ by piecing together the maps $\mathtt T_\mathfrak a$. In this paper we show how the original idea of Sudakov can be successfully implemented. The results yield a complete characterization of the Kantorovich optimal transportation problem, whose straightforward corollary is the solution of the Monge problem in each set $Z_\mathfrak a$ and then in $\mathbb R^d$. The strategy is sufficiently powerful to be applied to other optimal transportation problems.