Abstract. The author uses gap-introducing and gap-preserving reductions and shows that for multiple reoptimization of the problem of calculating the chromatic number of a graph with a given exponential set of optimal solutions, when an arbitrary vertex with no more than two edges incident to it is inserted as well as when any vertex with all incident edges is deleted, polynomial time approximation scheme (PTAS) does not exist. The same result holds for ordinary reoptimization.

Keywords: multiple reoptimization, gap-introducing (gap-preserving) reductions, APX-difficulty, polynomial time approximation schemes (PTAS).