On Switching to H-Free Graphs


  • Results contained in this article appeared in conference versions [11] and [10].

  • Work on this article has been supported by Czech Ministry of Education research grant MSM0021620838 and CE-ITI project GACR P202/12/6061.


In this article, we study the problem of deciding if, for a fixed graph H, a given graph is switching equivalent to an H-free graph. Polynomial-time algorithms are known for H having at most three vertices or isomorphic to P4. We show that for H isomorphic to a claw, the problem is polynomial, too. On the other hand, we give infinitely many graphs H such that the problem is NP-complete, thus solving an open problem [Kratochvíl, Nešetřil and Zýka, Ann Discrete Math 51 (1992)]. Further, we give a characterization of graphs switching equivalent to a K1, 2-free graph by ten forbidden-induced subgraphs, each having five vertices. We also give the forbidden-induced subgraphs for graphs switching equivalent to a forest of bounded vertex degrees.