Shorter tours and longer detours: uniform covers and a bit beyond

Motivated by the well known “four-thirds conjecture” for the traveling salesman problem (TSP), we study the problem of uniform covers . A graph G = ( V , E ) has an α -uniform cover for TSP (2EC, respectively) if the everywhere α vector (i.e., { α } E ) dominates a convex combination of incidence vectors of tours (2-edge-connected spanning multigraphs, respectively). The polyhedral analysis of Christofides’ algorithm directly implies that a 3-edge-connected, cubic graph has a 1-uniform cover for TSP. Sebő asked if such graphs have ( 1 - ϵ ) -uniform covers for TSP for some ϵ > 0 . Indeed, the four-thirds conjecture implies that such graphs have 8 9 -uniform covers. We show that these graphs have 18 19 -uniform covers for TSP. We also study uniform covers for 2EC and show that the everywhere 15 17 vector can be efficiently written as a convex combination of 2-edge-connected spanning multigraphs. For a weighted, 3-edge-connected, cubic graph, our results show that if the everywhere 2 3 vector is an optimal solution for the subtour elimination linear programming relaxation for TSP, then a tour with weight at most 27 19 times that of an optimal tour can be found efficiently. Node-weighted , 3-edge-connected, cubic graphs fall into this category. In this special case, we can apply our tools to obtain an even better approximation guarantee. An essential ingredient in our proofs is decompositions of graphs (e.g., cycle covers) that cover small-cardinality cuts an even (nonzero) number of times. Another essential tool we use is half-integral tree augmentation, which is known to have a small integrality gap. To extend our approach to input graphs that are 2-edge-connected, we present a procedure to decompose a point in the subtour elimination polytope into spanning, connected subgraphs that cover each 2-edge cut an even number of times. Using this decomposition, we obtain a 17 12 -approximation algorithm for minimum weight 2-edge-connected spanning subgraphs on subcubic, node-weighted graphs.