Contracting Edges to Destroy a Pattern: A Complexity Study

Given a graph G and an integer k, the objective of the Pi-CONTRACTION problem is to check whether there exists at most k edges in G such that contracting them in G results in a graph satisfying the property Pi. We investigate the problem where Pi is 'H-free' (without any induced copies of H). It is trivial that H-FREE CONTRACTION is polynomial-time solvable if H is a complete graph of at most two vertices. We prove that, in all other cases, the problem is NP-complete. We then investigate the fixed-parameter tractability of these problems. We prove that whenever H is a tree, except for seven trees, H-FREE CONTRACTION is W[2]-hard. This result along with the known results leaves behind only three unknown cases among trees.