Maximum $Δ$-edge-colorable subgraphs of class II graphs

A graph $G$ is class II, if its chromatic index is at least $Δ+1$. Let $H$ be a maximum $Δ$-edge-colorable subgraph of $G$. The paper proves best possible lower bounds for $\frac{|E(H)|}{|E(G)|}$, and structural properties of maximum $Δ$-edge-colorable subgraphs. It is shown that every set of vertex-disjoint cycles of a class II graph with $Δ\geq3$ can be extended to a maximum $Δ$-edge-colorable subgraph. Simple graphs have a maximum $Δ$-edge-colorable subgraph such that the complement is a matching. Furthermore, a maximum $Δ$-edge-colorable subgraph of a simple graph is always class I.

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