On graphs with $1$-matching and $2$-matching edges

Abstract

Let \(G\) be a graph admitting a perfect matching. An edge is called a {\it \(k\)-matching edge} if it belongs to exactly \(k\) perfect matchings, and a {\it \(k^{+}\)-matching edge} if it belongs to at least \(k\) perfect matchings. Thus, {\it an admissible edge} is a \(1^{+}\)-matching edge, and a connected graph is {\it matching covered} if every edge is admissible. We call a connected graph {\it \(k\)-matching covered} if every edge is a \(k\)-matching edge; in particular, a \(2\)-matching covered graph is called {\it matching double covered}. Motivated by matching-covered graph theory and the Berge--Fulkerson conjecture (1970s), we introduce the class \(\mathfrak{B}\) of connected graphs in which every edge is either a \(1\)-matching edge or a \(2\)-matching edge, and no perfect matching contains edges of both types. In particular, every matching double covered graph belongs to \(\mathfrak{B}\). Using ear decompositions and tight-cut decompositions, we establish a complete structural characterization of graphs in \(\mathfrak{B}\). These characterizations reveal how restrictions on the number of perfect matchings containing each edge determine the global structure of the corresponding matching-covered graphs.