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Solving Hamiltonian Cycle by an EPT Algorithm for a Non-sparse Parameter
Many hard graph problems, such as Hamiltonian Cycle, become FPT when
parameterized by treewidth, a parameter that is bounded only on sparse graphs.
When parameterized by the more general parameter clique-width, Hamiltonian
Cycle becomes W[1]-hard, as shown by Fomin et al. [5]. S{\ae}ther and Telle
address this problem in their paper [13] by introducing a new parameter,
split-matching-width, which lies between treewidth and clique-width in terms of
generality. They show that even though graphs of restricted
split-matching-width might be dense, solving problems such as Hamiltonian Cycle
can be done in FPT time.
Recently, it was shown that Hamiltonian Cycle parameterized by treewidth is
in EPT [1, 6], meaning it can be solved in -time. In this
paper, using tools from [6], we show that also parameterized by
split-matching-width Hamiltonian Cycle is EPT. To the best of our knowledge,
this is the first EPT algorithm for any "globally constrained" graph problem
parameterized by a non-trivial and non-sparse structural parameter. To
accomplish this, we also give an algorithm constructing a branch decomposition
approximating the minimum split-matching-width to within a constant factor.
Combined, these results show that the algorithms in [13] for Edge Dominating
Set, Chromatic Number and Max Cut all can be improved. We also show that for
Hamiltonian Cycle and Max Cut the resulting algorithms are asymptotically
optimal under the Exponential Time Hypothesis