On the integrality gap of small Asymmetric Traveling Salesman Problems: A polyhedral and computational approach

In this paper, we investigate the integrality gap of the Asymmetric Traveling Salesman Problem (ATSP) with respect to the linear relaxation given by the Asymmetric Subtour Elimination Problem (ASEP) for instances with $n$ nodes, where $n$ is small. In particular, we focus on the geometric properties and symmetries of the ASEP polytope ($P_{\mathrm{ASEP}}^n$) and its vertices. The polytope’s symmetries are exploited to design a heuristic pivoting algorithm to search vertices where the integrality gap is maximized. Furthermore, a general procedure for the extension of vertices from $P_{\mathrm{ASEP}}^n$ to $P_{\mathrm{ASEP}}^{n+1}$ is defined. The generated vertices improve the known lower bounds of the integrality gap for $16\le n\le 22$ and, provide small hard-to-solve ATSP instances.