The Parameterized Complexity of Fixing Number and Vertex Individualization in Graphs

In this paper we study the algorithmic complexity of the following problems: (1) Given a vertex-colored graph X = (V,E,c), compute a minimum cardinality set of vertices S⊆ V such that no nontrivial automorphism of X fixes all vertices in S. A closely related problem is computing a minimum base S for a permutation group G ≤ Sym(n) given by generators, i.e., a minimum cardinality subset S ⊆ [n] such that no nontrivial permutation in G fixes all elements of S. Our focus is mainly on the parameterized complexity of these problems. We show that when k=|S| is treated as parameter, then both problems are MINI[1]-hard. For the dual problems, where k = n – |S| is the parameter, we give FPT algorithms.(2) A notion related to fixing is individualization, which is a useful technique combined with the Weisfeiler-Leman procedure in algorithms for Graph Isomorphism. We explore the complexity of individualization: the problem of computing the minimum number of vertices we need to individualize in a given graph such that color refinement results in a graph with useful structural properties in the context of Graph Isomorphism and the Weisfeiler-Leman procedure.