Extremal Threshold Graphs for Matchings and Independent Sets

TL;DR

This paper investigates extremal properties of threshold graphs, identifying those that maximize matchings and minimize independent sets, and introduces the concept of almost alternating threshold graphs as extremal examples.

Contribution

It characterizes threshold graphs that maximize matchings and minimize independent sets, introducing almost alternating threshold graphs as extremal structures.

Findings

Almost alternating threshold graphs minimize matchings.

Threshold graphs with fewest independent sets identified.

Extremal threshold graphs differ from classical lex or colex graphs.

Abstract

Many extremal problems for graphs have threshold graphs as their extremal examples. For instance the current authors proved that for fixed $k\ge 1$, among all graphs on $n$ vertices with $m$ edges, some threshold graph has the fewest matchings of size $k$; indeed either the lex graph or the colex graph is such an extremal example. In this paper we consider the problem of maximizing the number of matchings in the class of threshold graphs. We prove that the minimizers are what we call \emph{almost alternating threshold graphs}. We also discuss a problem with a similar flavor: which threshold graph has the fewest independent sets. Here we are inspired by the result that among all graphs on $n$ vertices and $m$ edges the lex graph has the most independent sets.