Constant-Factor Approximations for the Biased Traveling Salesman Problem and Its Path Variant

Abstract

The metric traveling salesman problem (TSP) is a fundamental problem in combinatorial optimization that asks for a minimum-cost Hamiltonian cycle in a complete undirected graph with $N$ vertices, where edge costs satisfy the triangle inequality. The biased TSP (BTSP) is a natural generalization of the metric TSP, in which the vertex set is partitioned into $k$ subsets: edge costs satisfy the triangle inequality within each subset, while edges crossing the subsets may violate the triangle inequality by a certain bias factor. This model captures scenarios in which distances are metric only locally but may be distorted across different regions or types of vertices. In this paper, we study approximation algorithms for the BTSP and its path variant, the biased traveling salesman path problem (BTSPP), which asks for a minimum-cost Hamiltonian path. Previously, constant-factor approximation algorithms were known only for the case $k=2$. Whether the BTSP and the BTSPP admit constant-factor approximation algorithms for any fixed integer $k>2$ has remained an open question. We resolve this question affirmatively. We show that both the BTSP and the BTSPP admit $O(k)$-approximation algorithms with running time $N^{O(2^k k!)}$. Moreover, for the BTSPP with $k=2$, we propose an improved $(α+2+\varepsilon)$-approximation algorithm, where $α<3/2-10^{-34}$ is the best-known approximation ratio for the metric TSP, improving upon the previous ratio of $4$