# On the complexity of k-rainbow cycle colouring problems

## TL;DR

This paper investigates the computational complexity of k-rainbow cycle colouring problems, establishing polynomial-time solvability for some cases and NP-Completeness for others, highlighting the difficulty of certain decision problems in graph colourings.

## Contribution

It provides a comprehensive complexity analysis of k-rainbow cycle colouring problems, identifying which cases are polynomial-time solvable and which are NP-Complete.

## Key findings

- Deciding crx_1=3 is polynomial-time solvable.
- Deciding crx_1 ≤ k for k ≥ 4 is NP-Complete.
- Deciding crx_2=3 is polynomial-time solvable.

## Abstract

An edge-coloured cycle is $rainbow$ if all edges of the cycle have distinct colours. For $k\geq 1$, let $\mathcal{F}_{k}$ denote the family of all graphs with the property that any $k$ vertices lie on a cycle. For $G\in \mathcal{F}_{k}$, a $k$-$rainbow$ $cycle$ $colouring$ of $G$ is an edge-colouring such that any $k$ vertices of $G$ lie on a rainbow cycle in $G$. The $k$-$rainbow$ $cycle$ $index$ of $G$, denoted by $crx_{k}(G)$, is the minimum number of colours needed in a $k$-rainbow cycle colouring of $G$. In this paper, we restrict our attention to the computational aspects of $k$-rainbow cycle colouring. First, we prove that the problem of deciding whether $crx_1=3$ can be solved in polynomial time, but that of deciding whether $crx_1 \leq k$ is NP-Complete, where $k\geq 4$. Then we show that the problem of deciding whether $crx_2=3$ can be solved in polynomial time, but those of deciding whether $crx_2 \leq 4$ or $5$ are NP-Complete. Furthermore, we also consider the cases of $crx_3=3$ and $crx_3 \leq 4$. Finally, We prove that the problem of deciding whether a given edge-colouring (with an unbounded number of colours) of a graph is a $k$-rainbow cycle colouring, is NP-Complete for $k=1$, $2$ and $3$, respectively. Some open problems for further study are mentioned.