Uniqueness for the Skorokhod problem in an orthant: critical cases

TL;DR

This paper investigates the conditions for uniqueness in the Skorokhod problem within an orthant, extending known results to critical spectral radius cases and resolving open questions in two dimensions.

Contribution

It proves pathwise uniqueness when the spectral radius equals one and settles open cases for the deterministic problem in two dimensions.

Findings

Pathwise uniqueness holds at spectral radius 1.

Open cases for the deterministic problem in 2D are resolved.

Provides conditions for uniqueness in critical spectral radius cases.

Abstract

Consider the Skorokhod problem in the closed non-negative orthant: find a solution $(g(t),m(t))$ to \[ g(t)= f(t)+ Rm(t),\] where $f$ is a given continuous vector-valued function with $f(0)$ in the orthant, $R$ is a given $d\times d$ matrix with 1's along the diagonal, $g$ takes values in the orthant, and $m$ is a vector-valued function that starts at 0, each component of $m$ is non-decreasing and continuous, and for each $i$ the $i^{th}$ coordinate of $m$ increases only when the $i^{th}$ coordinate of $g$ is 0. The stochastic version of the Skorokhod problem replaces $f$ by the paths of Brownian motion. It is known that there exists a unique solution to the Skorokhod problem if the spectral radius of $|Q|$ is less than 1, where $Q=I-R$ and $|Q|$ is the matrix whose entries are the absolute values of the corresponding entries of $Q$. The first result of this paper shows pathwise uniqueness for the stochastic version of the Skorokhod problem holds if the spectral radius of $|Q|$ is equal to 1. The second result of this paper settles the remaining open cases for uniqueness for the deterministic version when the dimension $d$ is two.