,
Indu Ramesh
Creative Commons Attribution 4.0 International license
A large body of work studies the problem of learning an approximation to an implicit matrix 𝐀 ∈ ℝ^{m×n} that is only accessible implicitly via matrix-vector product queries (matvec queries) of the form 𝐱 → 𝐀𝐱 or 𝐱 → 𝐀^T𝐱. Of particular interest are methods that learn a near-optimal approximation with a fixed sparsity pattern. For example, we might want to learn a near-optimal diagonal, banded, or arrow-head approximation to an implicit matrix 𝐀.
Naturally, the number of matvec queries required to solve this problem depends on the sparsity pattern, which can be encoded as a binary matrix 𝐒 ∈ {0,1}^{m× n}. The query complexity of previous algorithms scales with quantities like the total number of ones in 𝐒, its maximum column/row sparsity, or the chromatic number of its "conflict graph". These quantities are incomparable: for a given 𝐒, parameterizing by one might yield lower query complexity than another.
In this work, we unify and tighten these prior results by providing a nearly sharp characterization of the matvec query complexity of sparse matrix approximation. Generalizing a definition from graph algorithms, let the degeneracy, degen(𝐒), denote the smallest number k so that, if we iteratively delete all rows and columns of 𝐒 with ≤ k ones, we are left with an empty matrix. We show that a near-optimal approximation to 𝐀 with sparsity pattern 𝐒 can be learned with Õ(degen(𝐒)) matrix-vector product queries, and Ω(degen(𝐒)) queries are necessary, for any sparsity pattern 𝐒. Moreover, unlike prior work based on graph coloring, all of our methods run in polynomial time.
@InProceedings{musco_et_al:LIPIcs.ESA.2026.85,
author = {Musco, Christopher and Ramesh, Indu},
title = {{Nearly Instance Optimal Sparse Matrix Approximation from Matrix-Vector Products}},
booktitle = {34th Annual European Symposium on Algorithms (ESA 2026)},
pages = {85:1--85:14},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-95977-445-1},
ISSN = {1868-8969},
year = {2026},
volume = {388},
editor = {Bille, Philip and Pettie, Seth and Storandt, Sabine},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.ESA.2026.85},
URN = {urn:nbn:de:0030-drops-272213},
doi = {10.4230/LIPIcs.ESA.2026.85},
annote = {Keywords: Matrix learning, sparse approximation, implicit methods}
}