A Box-Constrained Approach for Hard Permutation Problems

We describe the use of sorting networks to form relaxations of problems involving permutations of n objects. This approach is an alternative to relaxations based on the Birkhoff polytope (the set of n × n doubly stochastic matrices), providing a more compact formulation in which the only constraints are box constraints. Using this approach, we form a variant of the relaxation of the quadratic assignment problem recently studied in Vogelstein et al. (2015), and show that the continuation method applied to this formulation can be quite effective. We develop a coordinate descent algorithm that achieves a per-cycle complexity of O(n log n). We compare this method with Fast Approximate QAP (FAQ) algorithm introduced in Vogelstein et al. (2015), which uses a conditional-gradient method whose per-iteration complexity is O(n). We demonstrate that for most problems in QAPLIB and for a class of synthetic QAP problems, the sorting-network formulation returns solutions that are competitive with the FAQ algorithm, often in significantly less computing time.