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The treedepth of a graph G is the least possible depth of an elimination forest of G: a rooted forest on the same vertex set where every pair of vertices adjacent in G is bound by the ancestor/descendant relation. We propose an algorithm that given a graph G and an integer d, either finds an elimination forest of G of depth at most d or concludes that no such forest exists; thus the algorithm decides whether the treedepth of G is at most d. The running time is 2^𝒪(d²)⋅n^𝒪(1) and the space usage is polynomial in n. Further, by allowing randomization, the time and space complexities can be improved to 2^𝒪(d²)⋅n and d^𝒪(1)⋅n, respectively. This improves upon the algorithm of Reidl et al. [ICALP 2014], which also has time complexity 2^𝒪(d²)⋅n, but uses exponential space.
@InProceedings{nadara_et_al:LIPIcs.ESA.2022.79,
author = {Nadara, Wojciech and Pilipczuk, Micha{\l} and Smulewicz, Marcin},
title = {{Computing Treedepth in Polynomial Space and Linear FPT Time}},
booktitle = {30th Annual European Symposium on Algorithms (ESA 2022)},
pages = {79:1--79:14},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-247-1},
ISSN = {1868-8969},
year = {2022},
volume = {244},
editor = {Chechik, Shiri and Navarro, Gonzalo and Rotenberg, Eva and Herman, Grzegorz},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2022.79},
URN = {urn:nbn:de:0030-drops-170175},
doi = {10.4230/LIPIcs.ESA.2022.79},
annote = {Keywords: treedepth, FPT, polynomial space}
}