,
Karthik Gajulapalli
,
Alexander Golovnev
,
Evelyn Warton
Creative Commons Attribution 4.0 International license
We give two algorithms for output-sparse matrix multiplication (OSMM), the problem of multiplying two n × n matrices A, B when their product AB is promised to have at most O(n^δ) many non-zero entries for a given value δ ∈ [0, 2]. We then show how to speed up these algorithms in the fully sparse matrix multiplication (FSMM) setting, where the input matrices A, B are themselves sparse. All of our algorithms work over arbitrary rings.
Our first, deterministic algorithm for OSMM works via a two-pass reduction to compressed sensing. It runs in roughly n^{ω(δ/2, 1, 1)} time, where ω(⋅, ⋅, ⋅) is the rectangular matrix multiplication exponent. This substantially improves on prior deterministic algorithms for output-sparse matrix multiplication.
Our second, randomized algorithm for OSMM works via a reduction to compressed sensing and a variant of matrix multiplication verification, and runs in roughly n^{ω(δ-1, 1, 1)} time. This algorithm and its extension to the fully sparse setting have running times that match those of the (randomized) algorithms for OSMM and FSMM, respectively, in recent work of Abboud, Bringmann, Fischer, and Künnemann (SODA, 2024). Our algorithm is quite simple when taking a suitable compressed sensing scheme as a black box.
@InProceedings{bennett_et_al:LIPIcs.APPROX/RANDOM.2026.40,
author = {Bennett, Huck and Gajulapalli, Karthik and Golovnev, Alexander and Warton, Evelyn},
title = {{Output-Sparse Matrix Multiplication Using Compressed Sensing}},
booktitle = {Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2026)},
pages = {40:1--40:20},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-449-9},
ISSN = {1868-8969},
year = {2026},
volume = {392},
editor = {Singh, Mohit and Gur, Tom},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX/RANDOM.2026.40},
URN = {urn:nbn:de:0030-drops-277570},
doi = {10.4230/LIPIcs.APPROX/RANDOM.2026.40},
annote = {Keywords: Matrix Multiplication, Sparse Matrices, Compressed Sensing}
}