文摘
Shen and Zheng (2010) and Xu and Yan (2013) considered the Monge-Kantorovich problem in the plane and proved that the optimal coupling for the problem has a form (, ), and then they assumed has a density and gave the equation which should satisfy. In this article, we prove that naturally has a density under more weak conditions. We again prove a similar result in dimension 3 and give an exact form (, ) depending on a certain convex function.