Maximum size of a triangle-free graph with bounded maximum degree and matching number

Abstract

Determining the maximum number of edges under degree and matching number constraints have been solved for general graphs in Chvátal and Hanson (J Combin Theory Ser B 20:128–138, 1976) and Balachandran and Khare (Discrete Math 309:4176–4180, 2009). It follows from the structure of those extremal graphs that deciding whether this maximum number decreases or not when restricted to claw-free graphs, to \(C_4\)-free graphs or to triangle-free graphs are separately interesting research questions. The first two cases being already settled in Dibek et al. (Discrete Math 340:927–934, 2017) and Blair et al. (Latin American symposium on theoretical informatics, 2020), in this paper we focus on triangle-free graphs. We show that unlike most cases for claw-free graphs and \(C_4\)-free graphs, forbidding triangles from extremal graphs causes a strict decrease in the number of edges and adds to the hardness of the problem. We provide a formula giving the maximum number of edges in a triangle-free graph with degree at most d and matching number at most m for all cases where \(d\ge m\), and for the cases where \(d<m\) with either \(d\le 6\) or \(Z(d)\le m < 2d\) where Z(d) is a function of d which is roughly 5d/4. We also provide an integer programming formulation for the remaining cases and as a result of further discussion on this formulation, we conjecture that our formula giving the size of triangle-free extremal graphs is also valid for these open cases.