Complexity of Cycle Length Modularity Problems in Graphs

TL;DR

This paper classifies the computational complexity of cycle length modularity problems in graphs, determining which are solvable in polynomial time and which are NP-complete, based on cycle length conditions.

Contribution

It provides a complete classification of the complexity of cycle length modularity problems for various parameters, filling a gap in understanding their computational difficulty.

Findings

Classified $(S,m)$-UC problems as polynomial or NP-complete.

Classified $(S,m)$-DC problems as polynomial or NP-complete.

Identified conditions for polynomial-time solvability when $0 otin S$.

Abstract

The even cycle problem for both undirected and directed graphs has been the topic of intense research in the last decade. In this paper, we study the computational complexity of \emph{cycle length modularity problems}. Roughly speaking, in a cycle length modularity problem, given an input (undirected or directed) graph, one has to determine whether the graph has a cycle $C$ of a specific length (or one of several different lengths), modulo a fixed integer. We denote the two families (one for undirected graphs and one for directed graphs) of problems by $(S,m)\hbox{-}{\rm UC}$ and $(S,m)\hbox{-}{\rm DC}$, where $m \in \mathcal{N}$ and $S \subseteq \{0,1, ..., m-1\}$. $(S,m)\hbox{-}{\rm UC}$ (respectively, $(S,m)\hbox{-}{\rm DC}$) is defined as follows: Given an undirected (respectively, directed) graph $G$, is there a cycle in $G$ whose length, modulo $m$, is a member of $S$? In this paper, we fully classify (i.e., as either polynomial-time solvable or as ${\rm NP}$-complete) each problem $(S,m)\hbox{-}{\rm UC}$ such that $0 \in S$ and each problem $(S,m)\hbox{-}{\rm DC}$ such that $0 \notin S$. We also give a sufficient condition on $S$ and $m$ for the following problem to be polynomial-time computable: $(S,m)\hbox{-}{\rm UC}$ such that $0 \notin S$.