Kernelization of Vertex Cover by Structural Parameters
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In the NP-complete problem Vertex Cover, one is given a graph G and an integer k and are asked whether there exists a vertex set S ⊆ V (G) with size at most k such that every edge of the graph is incident to a vertex in S. In this thesis we explore techniques to solve Vertex Cover using parameterized algorithms, with a particular focus on kernelization by structural parameters. We present two new polynomial kernels for Vertex Cover, one parameterized by the size of a minimum degree-2 modulator, and one parameterized by the size of a minimum pseudoforest modulator. A degree-2 modulator is a vertex set X ⊆ V (G) such that G-X has maximum degree two, and a pseudoforest modulator is a vertex set X ⊆ V (G) such that every connected component of G-X has at most one cycle. Specifically, we provide polynomial time algorithms that for an input graph G and an integer k, outputs a graph G' and an integer k' such that G has a vertex cover of size k if and only if G' has a vertex cover of size k'. Moreover, the number of vertices of G' is bounded by O(|X|^7) where |X| is the size of a minimum degree-2 modulator for G, or bounded by O(|X|^12) where |X| is the size a minimum pseudoforest modulator for G. Our result extends known results on structural kernelization for Vertex Cover.