Globally Solving Unbalanced Optimal Transport and Density Control for Gaussian Distributions

TL;DR

This paper develops a control-theoretic framework for unbalanced optimal transport and density control of Gaussian distributions, providing exact finite-dimensional solutions and practical algorithms.

Contribution

It introduces the unbalanced density control (UDC) method for Gaussian measures, extending optimal transport with control theory and deriving explicit solutions.

Findings

Exact Gaussian reduction for UOT with quadratic cost and KL penalties.

Finite-dimensional reformulation of UDC as SDP with closed-form mass update.

Existence of optimal solutions and conditions for deterministic affine-Gaussian policies.

Abstract

In this article, we study unbalanced optimal transport (UOT) and establish a control-theoretic dynamical extension, which we call the unbalanced density control (UDC), for a class of Gaussian reference measures. In the static setting, we consider UOT with quadratic transport cost and Kullback--Leibler penalties on the marginals relative to prescribed Gaussian measures. We show that the infinite-dimensional variational problem admits an exact Gaussian reduction, yielding a finite-dimensional optimization over masses, means, and covariances, together with a closed-form expression for the optimal transported mass. We then formulate UDC for discrete-time linear systems, where the initial and terminal state measures are imposed softly through KL penalties and the intermediate evolution is governed by controlled linear dynamics with quadratic control cost. For this problem, we prove that any feasible solution can be replaced, without loss of optimality, by a Gaussian initial measure and an affine-Gaussian control policy. This leads to an exact finite-dimensional reformulation and, after a standard covariance-steering lifting, to an SDP-based optimization for fixed mass, again coupled with a closed-form mass update. We further establish existence of optimal solutions and identify a sufficient condition under which the affine-Gaussian UDC policy is deterministic. These results provide globally optimal solution methods for both Gaussian UOT and Gaussian UDC. Finally, we illustrate our results with several numerical examples.