Critical Independent Sets of a Graph
  Let $G$ be a simple graph with vertex set $V\left( G\right) $. A set $S\subseteq V\left( G\right) $ is independent if no two vertices from $S$ are adjacent, and by $\mathrm{Ind}(G)$ we mean the family of all independent sets of $G$.   The number $d\left( X\right) =$ $\left\vert X\right\vert -\left\vert N(X)\right\vert $ is the difference of $X\subseteq V\left( G\right) $, and a set $A\in\mathrm{Ind}(G)$ is critical if $d(A)=\max \{d\left( I\right) :I\in\mathrm{Ind}(G)\}$ (Zhang, 1990).   Let us recall the following definitions:   $\mathrm{core}\left( G\right) $ = $\bigcap$ {S : S is a maximum independent set}.   $\mathrm{corona}\left( G\right)$ = $\bigcup$ {S :S is a maximum independent set}.   $\mathrm{\ker}(G)$ = $\bigcap$ {S : S is a critical independent set}.   $\mathrm{diadem}(G)$ = $\bigcup$ {S : S is a critical independent set}.   In this paper we present various structural properties of $\mathrm{\ker}(G)$, in relation with $\mathrm{core}\left( G\right) $, $\mathrm{corona}\left( G\right) $, and $\mathrm{diadem}(G)$. 
Vadim E. Levit and Eugen Mandrescu
cs.DM math.CO