Cut-edges and regular factors in regular graphs of odd degree

TL;DR

This paper generalizes a known result about the existence of 2-factors in regular graphs with cut-edges, establishing sharp bounds for the existence of 2k-factors in odd-degree regular graphs.

Contribution

It extends previous work by providing sharp bounds for the existence of 2k-factors in (2r+1)-regular graphs with limited cut-edges, and characterizes extremal graphs.

Findings

Established sharp bounds for 2k-factors in (2r+1)-regular graphs.

Characterized graphs with specific cut-edge counts lacking 2k-factors.

Extended known results to broader parameter ranges.

Abstract

We study $2k$-factors in $(2r+1)$-regular graphs. Hanson, Loten, and Toft proved that every $(2r+1)$-regular graph with at most $2r$ cut-edges has a $2$-factor. We generalize their result by proving for $k\le(2r+1)/3$ that every $(2r+1)$-regular graph with at most $2r-3(k-1)$ cut-edges has a $2k$-factor. Both the restriction on $k$ and the restriction on the number of cut-edges are sharp. We characterize the graphs that have exactly $2r-3(k-1)+1$ cut-edges but no $2k$-factor. For $k>(2r+1)/3$, there are graphs without cut-edges that have no $2k$-factor, as studied by Bollob\'as, Saito, and Wormald.