The k-CO-PATH SET problem asks, given a graph G and a positive integer k, whether one can delete k edges from G so that the remainder is a collection of disjoint paths. We give a linear-time, randomized fpt algorithm with complexity O^*(1.588^k) for deciding k-CO-PATH SET, significantly improving the previously best known O^*(2.17^k) of Feng, Zhou, and Wang (2015). Our main tool is a new O^*(4^{tw(G)}) algorithm for CO-PATH SET using the Cut&Count framework, where tw(G) denotes treewidth. In general graphs, we combine this with a branching algorithm which refines a 6k-kernel into reduced instances, which we prove have bounded treewidth.
@InProceedings{sullivan_et_al:LIPIcs.IPEC.2016.28, author = {Sullivan, Blair D. and van der Poel, Andrew}, title = {{A Fast Parameterized Algorithm for Co-Path Set}}, booktitle = {11th International Symposium on Parameterized and Exact Computation (IPEC 2016)}, pages = {28:1--28:13}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-023-1}, ISSN = {1868-8969}, year = {2017}, volume = {63}, editor = {Guo, Jiong and Hermelin, Danny}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.IPEC.2016.28}, URN = {urn:nbn:de:0030-drops-69371}, doi = {10.4230/LIPIcs.IPEC.2016.28}, annote = {Keywords: co-path set, parameterized complexity, Cut\&Count, bounded treewidth} }
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