Decycling cubic graphs
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A set of vertices of a graph G is said to be decycling if its removal leaves an acyclic subgraph. The size of a smallest decycling set is the decycling number of G. Generally, at least ⌈(n+2)/4⌉ vertices have to be removed in order to decycle a cubic graph on n vertices. In 1979, Payan and Sakarovitch proved that the decycling number of a cyclically 4-edge-connected cubic graph of order n equals ⌈ (n+2)/4⌉. In addition, they characterised the structure of minimum decycling sets and their complements. If n≡ 24, then G has a decycling set which is independent and its complement induces a tree. If n≡ 04, then one of two possibilities occurs: either G has an independent decycling set whose complement induces a forest of two trees, or the decycling set is near-independent (which means that it induces a single edge) and its complement induces a tree. In this paper we strengthen the result of Payan and Sakarovitch by proving that the latter possibility (a near-independent set and a tree) can always be guaranteed. Moreover, we relax the assumption of cyclic 4-edge-connectivity to a significantly weaker condition expressed through the canonical decomposition of 3-connected cubic graphs into cyclically 4-edge-connected ones. Our methods substantially use a surprising and seemingly distant relationship between the decycling number and the maximum genus of a cubic graph.
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