On the skeleton of the polytope of pyramidal tours

Abstract

We consider the skeleton of the polytope of pyramidal tours. A Hamiltonian tour is called pyramidal if the salesperson starts in city 1, then visits some cities in increasing order of their numbers, reaches city n, and returns to city 1 visiting the remaining cities in decreasing order. The polytope PYR(n) is defined as the convex hull of the characteristic vectors of all pyramidal tours in the complete graph K_n. The skeleton of PYR(n) is the graph whose vertex set is the vertex set of PYR(n) and the edge set is the set of geometric edges or one-dimensional faces of PYR(n). We describe the necessary and sufficient condition for the adjacency of vertices of the polytope PYR(n). On this basis we developed an algorithm to check the vertex adjacency with linear complexity. We establish that the diameter of the skeleton of PYR(n) equals 2, and the asymptotically exact estimate of the clique number of the skeleton of PYR(n) is Θ(n²). It is known that this value characterizes the time complexity in a broad class of algorithms based on linear comparisons.