An Optimal XP Algorithm for Hamiltonian Cycle on Graphs of Bounded Clique-Width

For MSO2-expressible problems like Edge Dominating Set or Hamiltonian Cycle, it was open for a long time whether there is an algorithm which given a clique-width k-expression of an n-vertex graph runs in time f(k) · nO(1) for some function f . Recently, Fomin et al. (SIAM. J. Computing, 2014) presented several lower bounds; for instance, there are no f(k) · n-time algorithms for Edge Dominating Set and for Hamiltonian Cycle unless the Exponential Time Hypothesis (ETH) fails. They also provided an algorithm running in time nO(k) for Edge Dominating Set, but left open whether Hamiltonian Cycle can be solved in time nO(k). In this paper, we prove that Hamiltonian Cycle can be solved in time nO(k). This improves the naive algorithm that runs in time nO(k ) by Espelage et al. (WG 2001). We present a general technique of representative sets using two-edge colored multigraphs on k vertices. The essential idea behind is that for a two-edge colored multigraph, the existence of an Eulerian trail that uses edges with different colors alternatively can be determined by two information, that are the number of colored edges incident with each vertex and the number of connected components containing an edge. This allows to avoid storing all possible graphs on k vertices with at most n edges, which gives the nO(k ) running time. We can apply this technique to other problems such as q-Cycle Covering or Directed Hamiltonian Cycle as well.