We consider the Monge-Kantorovich problem of transporting a probability density on R m to another on the line, so as to optimize a given cost function. We introduce a nestedness criterion relating the cost to the densities, under which it becomes possible to solve this problem uniquely, by constructing an optimal map one level set at a time. This map is continuous if the target density has connected support. We use level-set dynamics to develop and quantify a local regularity theory for this map and the Kantorovich potentials solving the dual linear program. We identify obstructions to global regularity through examples.More specifically, fix probability densities f and g on open sets X ⊂ R m and Y ⊂ R n with m ≥ n ≥ 1. Consider transporting f onto g so as to minimize the cost −s(x, y). We give a non-degeneracy condition (a) on s ∈ C 1,1 which ensures the set of * The authors are grateful to Toronto's Fields' Institute for the Mathematical Sciences for its kind hospitality during part of this work. RJM acknowledges partial support of this research by Natural Sciences and Engineering
1x paired with [g-a.e.] y ∈ Y lie in a codimension n submanifold of X. Specializing to the case m > n = 1, we discover a nestedness criteria relating s to (f, g) which allows us to construct a unique optimal solution in the form of a map F : X −→ Y . When s ∈ C 2 ∩ W 3,1 and log f and log g are bounded, the Kantorovich dual potentials (u, v) satisfy v ∈ C 1,1 loc (Y ), and the normal velocity V of F −1 (y) with respect to changes in y is given by V (x) = v ′′ (F (x)) − s yy (x, F (x)). Positivity (b) of V locally implies a Lipschitz bound on F ; moreover, v ∈ C 2 if F −1 (y) intersects ∂X ∈ C 1 transversally (c). On subsets where (a)-(c) can be be quantified, for each integer r ≥ 1 the norms of u, v ∈ C r+1,1 and F ∈ C r,1 are controlled by these bounds, log f, log g, ∂X C r−1,1 , ∂X C 1,1 , s C r+1,1 , and the smallness of F −1 (y). We give examples showing regularity extends from X to part of X, but not from Y to Y . We also show that when s remains nested for all (f, g), the problem in R m × R reduces to a supermodular problem in R × R.