Lift-and-Project Integrality Gaps for the Traveling Salesperson Problem

TL;DR

This paper investigates how lift-and-project procedures affect the integrality gaps of linear relaxations for various forms of the Traveling Salesperson Problem, providing new lower bounds after certain rounds of these procedures.

Contribution

It establishes new lower bounds on the integrality gaps of the TSP relaxation after one or multiple rounds of lift-and-project methods, extending previous results.

Findings

For asymmetric TSP, gap is at least 3/2 after one round.

For symmetric TSP, gap remains at least 4/3 after o(n) rounds.

For symmetric TSP path, gap remains at least 3/2 after o(n) rounds.

Abstract

We study the lift-and-project procedures of Lov{\'a}sz-Schrijver and Sherali-Adams applied to the standard linear programming relaxation of the traveling salesperson problem with triangle inequality. For the asymmetric TSP tour problem, Charikar, Goemans, and Karloff (FOCS 2004) proved that the integrality gap of the standard relaxation is at least 2. We prove that after one round of the Lov{\'a}sz-Schrijver or Sherali-Adams procedures, the integrality gap of the asymmetric TSP tour problem is at least 3/2, with a small caveat on which version of the standard relaxation is used. For the symmetric TSP tour problem, the integrality gap of the standard relaxation is known to be at least 4/3, and Cheung (SIOPT 2005) proved that it remains at least 4/3 after $o(n)$ rounds of the Lov{\'a}sz-Schrijver procedure, where $n$ is the number of nodes. For the symmetric TSP path problem, the integrality gap of the standard relaxation is known to be at least 3/2, and we prove that it remains at least 3/2 after $o(n)$ rounds of the Lov{\'a}sz-Schrijver procedure, by a simple reduction to Cheung's result.