Improved Approximation Algorithms for Capacitated Vehicle Routing with Fixed Capacity

TL;DR

This paper introduces improved approximation algorithms for the capacitated vehicle routing problem (CVRP) variants, achieving better ratios especially for small vehicle capacities, and extends techniques that could benefit other routing problems.

Contribution

The paper presents new approximation algorithms with improved ratios for all CVRP variants, especially for small capacities, and introduces the EX-ITP technique extending classic methods.

Findings

Achieved a 1.500 approximation ratio for k=3 in all CVRP variants.

Improved approximation ratios for small k, notably k=4 and k=5.

Developed the EX-ITP method, extending the classic ITP approach.

Abstract

The Capacitated Vehicle Routing Problem (CVRP) is one of the most extensively studied problems in combinatorial optimization. Based on customer demand, we distinguish three variants of CVRP: unit-demand, splittable, and unsplittable. In this paper, we consider $k$-CVRP in general metrics and on general graphs, where $k$ is the vehicle capacity. All three versions are APX-hard for any fixed $k\geq3$. Assume that the approximation ratio of metric TSP is $\frac{3}{2}$. We present a $(\frac{5}{2}-\Theta(\frac{1}{\sqrt{k}}))$-approximation algorithm for the splittable and unit-demand cases, and a $(\frac{5}{2}+\ln2-\Theta(\frac{1}{\sqrt{k}}))$-approximation algorithm for the unsplittable case. Our approximation ratio is better than the previous results when $k$ is less than a sufficiently large value, approximately $1.7\times10^7$. For small values of $k$, we design independent and elegant algorithms with further improvements. For the splittable and unit-demand cases, we improve the approximation ratio from $1.792$ to $1.500$ for $k=3$, and from $1.750$ to $1.500$ for $k=4$. For the unsplittable case, we improve the approximation ratio from $1.792$ to $1.500$ for $k=3$, from $2.051$ to $1.750$ for $k=4$, and from $2.249$ to $2.157$ for $k=5$. The approximation ratio for $k=3$ surprisingly achieves the same value as in the splittable case. Our techniques, such as EX-ITP -- an extension of the classic ITP method, have the potential to improve algorithms for other routing problems as well.