Finding, Hitting and Packing Cycles in Subexponential Time on Unit Disk Graphs

• Published in: Discrete & computational geometry Vol. 62; no. 4; pp. 879 - 911
• Main Authors: Fomin, Fedor V., Lokshtanov, Daniel, Panolan, Fahad, Saurabh, Saket, Zehavi, Meirav
• Format: Journal Article
• Language: English
• Published: New York Springer US 01.12.2019; Springer Nature B.V
• Subjects: Algorithms; Apexes; Combinatorics; Computational Mathematics and Numerical Analysis; Graphs; Markov analysis; Mathematics; Mathematics and Statistics; Run time (computers); Separators; Theorems; Longest path; 68Q25; 68W40; Feedback vertex set; 68W01; Cycle packing; Longest cycle; Unit disk graph; Parameterized complexity; Unit square graph
• ISSN: 0179-5376; 1432-0444
• DOI: 10.1007/s00454-018-00054-x

We give algorithms with running time 2 O ( k log k ) · n O ( 1 ) for the following problems. Given an n -vertex unit disk graph G and an integer k , decide whether G contains a path on exactly/at least k vertices, a cycle on exactly k vertices, a cycle on at least k vertices, a feedback vertex set of size at most k , and a set of k pairwise vertex-disjoint cycles. For the first three problems, no subexponential time parameterized algorithms were previously known. For the remaining two problems, our algorithms significantly outperform the previously best known parameterized algorithms that run in time 2 O ( k 0.75 log k ) · n O ( 1 ) . Our algorithms are based on a new kind of tree decompositions of unit disk graphs where the separators can have size up to k O ( 1 ) and there exists a solution that crosses every separator at most O ( k ) times. The running times of our algorithms are optimal up to the log k factor in the exponent, assuming the exponential time hypothesis.