# Linear transformations between colorings in chordal graphs

## TL;DR

This paper proves that for chordal graphs with certain colorings, there exists a linear-length recoloring sequence between any two colorings, and provides a linear-time algorithm to find such a sequence.

## Contribution

It establishes a linear upper bound on the recoloring sequence length for chordal graphs when the number of colors is at least four more than the degeneracy, with a constructive proof and algorithm.

## Key findings

- Existence of linear-length recoloring sequences for chordal graphs with k ≥ d+4.
- A linear-time algorithm to compute the recoloring sequence.
- Improvement over previous exponential and quadratic bounds.

## Abstract

Let $k$ and $d$ be such that $k \ge d+2$. Consider two $k$-colorings of a $d$-degenerate graph $G$. Can we transform one into the other by recoloring one vertex at each step while maintaining a proper coloring at any step? Cereceda et al. answered that question in the affirmative, and exhibited a recolouring sequence of exponential length. If $k=d+2$, we know that there exists graphs for which a quadratic number of recolorings is needed. And when $k=2d+2$, there always exists a linear transformation. In this paper, we prove that, as long as $k \ge d+4$, there exists a transformation of length at most $f(\Delta) \cdot n$ between any pair of $k$-colorings of chordal graphs (where $\Delta$ denotes the maximum degree of the graph). The proof is constructive and provides a linear time algorithm that, given two $k$-colorings $c_1,c_2$ computes a linear transformation between $c_1$ and $c_2$.