Polar factorization and monotone rearrangement of vector‐valued functions

Abstract Given a probability space ( X , μ) and a bounded domain Ω in ℝ d equipped with the Lebesgue measure |·| (normalized so that |Ω| = 1), it is shown (under additional technical assumptions on X and Ω) that for every vector‐valued function u ∈ L p ( X , μ; ℝ d ) there is a unique “polar factorization” u = ∇Ψ s , where Ψ is a convex function defined on Ω and s is a measure‐preserving mapping from ( X , μ) into (Ω, |·|), provided that u is nondegenerate, in the sense that μ( u −1 ( E )) = 0 for each Lebesgue negligible subset E of ℝ d . Through this result, the concepts of polar factorization of real matrices, Helmholtz decomposition of vector fields, and nondecreasing rearrangements of real‐valued functions are unified. The Monge‐Ampère equation is involved in the polar factorization and the proof relies on the study of an appropriate “Monge‐Kantorovich” problem.