
Testing if a given graph $G$ contains the $k$-vertex path $P_k$ as a minor or as an induced minor is trivial for every fixed integer $k\geq 1$. However, the situation changes for the problem of checking if a graph can be modified into $P_k$ by using only edge contractions. In this case the problem is known to be NP-complete even if $k=4$. This led to an intensive investigation for testing contractibility on restricted graph classes. We focus on bipartite graphs. Heggernes, van 't Hof, L��v��que and Paul proved that the problem stays NP-complete for bipartite graphs if $k=6$. We strengthen their result from $k=6$ to $k=5$. We also show that the problem of contracting a bipartite graph to the $6$-vertex cycle $C_6$ is NP-complete. The cyclicity of a graph is the length of the longest cycle the graph can be contracted to. As a consequence of our second result, determining the cyclicity of a bipartite graph is NP-hard.
9 pages, 2 figures
FOS: Computer and information sciences, Computer Science - Computational Complexity, Discrete Mathematics (cs.DM), FOS: Mathematics, Mathematics - Combinatorics, Combinatorics (math.CO), Computational Complexity (cs.CC), Computer Science - Discrete Mathematics
FOS: Computer and information sciences, Computer Science - Computational Complexity, Discrete Mathematics (cs.DM), FOS: Mathematics, Mathematics - Combinatorics, Combinatorics (math.CO), Computational Complexity (cs.CC), Computer Science - Discrete Mathematics
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