# Problems on Matchings and Independent Sets of a Graph

## TL;DR

This paper investigates the properties of critical and minimal independent sets in graphs, proves a conjecture relating minimal sets to the critical difference, and explores structural characterizations of certain graph classes.

## Contribution

It proves a conjecture by Levit and Mandrescu on minimal sets with positive difference and provides new insights into the structure of unicyclic non-K"onig-Egerváry graphs.

## Key findings

- Confirmed the conjecture that minimal sets with positive difference are at least the critical difference in number.
- Provided a short proof of the inequality relating ker(G), diadem(G), and independence number.
- Characterized unicyclic non-K"onig-Egerváry graphs and proved a conjecture about their critical difference.

## Abstract

Let $G$ be a finite simple graph. For $X \subset V(G)$, the difference of $X$, $d(X) := |X| - |N (X)|$ where $N(X)$ is the neighborhood of $X$ and $\max \, \{d(X):X\subset V(G)\}$ is called the critical difference of $G$. $X$ is called a critical set if $d(X)$ equals the critical difference and ker$(G)$ is the intersection of all critical sets. It is known that ker$(G)$ is an independent (vertex) set of $G$. diadem$(G)$ is the union of all critical independent sets. An independent set $S$ is an inclusion minimal set with $d(S) > 0$ if no proper subset of $S$ has positive difference. A graph $G$ is called K\"onig-Egerv\'ary if the sum of its independence number ($\alpha (G)$) and matching number ($\mu (G)$) equals $|V(G)|$. It is known that bipartite graphs are K\"onig-Egerv\'ary. In this paper, we study independent sets with positive difference for which every proper subset has a smaller difference and prove a result conjectured by Levit and Mandrescu in 2013. The conjecture states that for any graph, the number of inclusion minimal sets $S$ with $d(S) > 0$ is at least the critical difference of the graph. We also give a short proof of the inequality $|$ker$(G)| + |$diadem$(G)| \le 2\alpha (G)$ (proved by Short in 2016). A characterization of unicyclic non-K\"onig-Egerv\'ary graphs is also presented and a conjecture which states that for such a graph $G$, the critical difference equals $\alpha (G) - \mu (G)$, is proved. We also make an observation about ker$G)$ using Edmonds-Gallai Structure Theorem as a concluding remark.