Down-left graphs and a connection to toric ideals of graphs

Abstract

We introduce a family of graphs, which we call down-left graphs, and study their combinatorial and algebraic properties. We show that members of this family are well-covered, $C_5$-free, and vertex decomposable. By applying a result of H\`a-Woodroofe and Moradi--Khosh-Ahang, the (Castelnuovo-Mumford) regularity of the associated edge ideals is the induced matching number of the graph. As an application, we give a combinatorial interpretation for the regularity of the toric ideals of chordal bipartite graphs that are $(K_{3,3} \setminus e)$-free.