As we have observed, p(G, r) = 0 whenever r < &>(G). So, suppose r >
Ù)(G) and assume / : V(G) -> C is a proper r-coloring. For each c e C, define
f~x(c) = [v eV(G): f(v) = c}. The co/or C/OM, / _ 1 ( C ) , consists of those
vertices of G to which / assigns the color c. From the definition of proper
coloring, / ~ ' ( c ) is an independent set. Therefore, o(/~'(c)) < a{G), c € C. No
single color can be shared by more than a(G) vertices. Because V(G) is the
disjoint union of the color classes,