# Towards Improving Christofides Algorithm on Fundamental Classes by Gluing Convex Combinations of Tours

## TL;DR

This paper introduces a novel method for improving Christofides algorithm on specific TSP instances by gluing tours over 3-edge cuts, reducing edge usage and achieving better approximation ratios.

## Contribution

It develops a new approach for gluing tours over 3-edge cuts in TSP, leading to improved bounds and a new approximation algorithm for certain problem instances.

## Key findings

- Reduced usage of edges with x-value 1 from 3/2 to 3/2 - θ/10.
- Constructed convex combinations of tours for specific fractional points.
- Achieved a 17/12-approximation for TSP under certain conditions.

## Abstract

We present a new approach for gluing tours over certain tight, 3-edge cuts. Gluing over 3-edge cuts has been used in algorithms for finding Hamilton cycles in special graph classes and in proving bounds for 2-edge-connected subgraph problem, but not much was known in this direction for gluing connected multigraphs. We apply this approach to the traveling salesman problem (TSP) in the case when the objective function of the subtour elimination relaxation is minimized by a $\theta$-cyclic point: $x_e \in \{0,\theta, 1-\theta, 1\}$, where the support graph is subcubic and each vertex is incident to at least one edge with $x$-value 1. Such points are sufficient to resolve TSP in general. For these points, we construct a convex combination of tours in which we can reduce the usage of edges with $x$-value 1 from the $\frac{3}{2}$ of Christofides algorithm to $\frac{3}{2}-\frac{\theta}{10}$ while keeping the usage of edges with fractional $x$-value the same as Christofides algorithm. A direct consequence of this result is for the Uniform Cover Problem for TSP: In the case when the objective function of the subtour elimination relaxation is minimized by a $\frac{2}{3}$-uniform point: $x_e \in \{0, \frac{2}{3}\}$, we give a $\frac{17}{12}$-approximation algorithm for TSP. For such points, this lands us halfway between the approximation ratios of $\frac{3}{2}$ of Christofides algorithm and $\frac{4}{3}$ implied by the famous "four-thirds conjecture".