Transport distances and geodesic convexity for systems of degenerate diffusion equations

Abstract: We introduce Wasserstein-like dynamical transport distances between vector-valued densities on the real line. The mobility function from the scalar theory is replaced by a mobility matrix, that is subject to positivity and concavity conditions. Our primary motivation is to cast certain systems of nonlinear parabolic evolution equations into the variational framework of gradient flows. In the first part of the paper, we investigate the structural properties of the new class of distances like geodesic completeness. The second part is devoted to the identification of $\lambda$-geodesically convex functionals and their $\lambda$-contractive gradient flows. One of our results is a generalized McCann condition for geodesic convexity of the internal energy. In the third part, the existence of weak solutions to a certain class of degenerate diffusion systems with drift is shown. Even if the underlying energy function is not geodesically convex w.r.t. our new distance, the construction of a weak solution is still possible using de Giorgi's minimizing movement scheme.