## Section: Research Program

### Optimal Transport

Given two measures, and calling transport maps the maps that transport the first measure into the second one, the Monge-Kantorovich problem of Optimal Transport is the search of the minimum of some cost on the set of transport maps. The cost of a map usually comes from some point to point cost and the transporte measure. This topic attracted renewed attention in the last decade, and has ongoing applications of many types, see section 4.5. Matching optimal transport with geometric control theory is one originality of our team. Work in the team has been concerned with optimal transport originating from Riemannian geometry ( [61] gives strong conditions for continuity of the transport map, [38], [54] checks these conditions on ellipsoids, [60] studies them on more general Riemannian manifolds), sub-Riemannian geometry (see section 6.2.2 and Zeinab Badreddine's PhD) or more general optimal control costs [64].

Let us sketch an important class of open problems. In collaboration with R. McCann [65], we worked towards identifying the costs that admit unique optimizers in the Monge-Kantorovich problem of optimal transport between arbitrary probability densities. For smooth costs and densities on compact manifolds, the only known examples for which the optimal solution is always unique require at least one of the two underlying spaces to be homeomorphic to a sphere. We have introduced a multivalued dynamics induced by the transportation cost between the target and source space, for which the presence or absence of a sufficiently large set of periodic trajectories plays a role in determining whether or not optimal transport is necessarily unique. This insight allows us to construct smooth costs on a pair of compact manifolds with arbitrary topology, so that the optimal transport between any pair of probability densities is unique. We investigated further this problem of uniquely minimizing costs and obtained in collaboration with Abbas Moameni [24] a result of density of uniquely minimizing costs in the ${C}^{0}$-topology. The results in higher topology should be the subject on some further research.