Sparse Feedback Implementation for Sender-Receiver Transportation Linear-Quadratic Control

Abstract

We study a sparse linear-quadratic problem for transportation dynamics. The sparsity pattern has a natural directed-graph representation in which vertices are storage locations and edges are transportation links. The goal is to compute the optimal control signal without applying the usually dense optimal feedback gain directly. We show that the optimal feedback gain can be factorized as the product of a sparse matrix and the inverse of another sparse matrix from the right. In contrast to an existing factorization that uses an inverse from the left, the proposed factors use graph-adapted state coordinates. When the underlying graph is a tree, we give, for every edge orientation, a closed formula for the Riccati matrix that determines these factors and a graph-based bound on their possible nonzero entries. This factorization can reduce the online cost relative to dense feedback multiplication and permits distributed evaluation. The main message is that linear quadratic control need not appear dense when expressed in graph-adapted coordinates, and that these coordinates can reveal intrinsic sparsity.