,
Aaron Putterman
,
Junkai Song
Creative Commons Attribution 4.0 International license
We study the parallel complexity of finding a basis of a graphic matroid under independence-oracle access. Karp, Upfal, and Wigderson (FOCS 1985, JCSS 1988) initiated the study of this problem and established two algorithms for finding a spanning forest: one running in O(log m) rounds with m^{Θ(log m)} queries, and another, for any d ∈ ℤ^+, running in O(m^{2/d}) rounds with Θ(m^d) queries. A key open question they posed was whether one could simultaneously achieve polylogarithmic rounds and polynomially many queries.
We give a deterministic algorithm that uses O(log m) adaptive rounds and poly(m) non-adaptive queries per round to return a spanning forest on m edges, and complement this result with a matching Ω(log m) lower bound for any (even randomized) algorithm with poly(m) queries per round. Thus, the adaptive round complexity for graphic matroids is characterized exactly, settling this long-standing problem.
Beyond graphs, we show that our framework also yields an O(log m)-round, poly(m)-query algorithm for any binary matroid satisfying a smooth circuit counting property, implying, among others, an optimal O(log m)-round parallel algorithms for finding bases of cographic matroids. Finally, we conjecture a natural strengthening of known circuit-counting bounds for the much broader class of regular matroids and even an extension to so-called max-flow min-cut matroids; assuming it, our algorithm achieves the same O(log m) rounds and poly(m) queries for all such matroids - which includes graphic and cographic matroids as special cases.
@InProceedings{khanna_et_al:LIPIcs.ICALP.2026.125,
author = {Khanna, Sanjeev and Putterman, Aaron and Song, Junkai},
title = {{Optimal Parallel Basis Finding in Graphic and Related Matroids}},
booktitle = {53rd International Colloquium on Automata, Languages, and Programming (ICALP 2026)},
pages = {125:1--125:22},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-428-4},
ISSN = {1868-8969},
year = {2026},
volume = {374},
editor = {Bhattacharya, Sayan and Nanongkai, Danupon and Benedikt, Michael and Puppis, Gabriele},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ICALP.2026.125},
URN = {urn:nbn:de:0030-drops-265143},
doi = {10.4230/LIPIcs.ICALP.2026.125},
annote = {Keywords: parallel algorithms, matroids}
}