Many challenging problems in graph theory involve at least one of the
following four invariants:
(i) the stability number a (G) (also called the independence number),
defined as the largest number of pairwise nonadjacent vertices in G;
(ii) the clique covering number 8(G), defined as the least number of cliques
which cover all the vertices of G;
(iii) the clique number w(G), defined as the largest number of pairwise
adjacent vertices in G;
(iv) the chromatic number y(G) (sometimes denoted also by x(G)), defined
as the least number of colors needed to color all the vertices in such a way that
no two adjacent vertices have the same color.