In this paper we study a worst case to average case reduction for the problem of matrix multiplication over finite fields. Suppose we have an efficient average case algorithm, that given two random matrices A,B outputs a matrix that has a non-trivial correlation with their product A ⋅ B. Can we transform it into a worst case algorithm, that outputs the correct answer for all inputs without incurring a significant overhead in the running time? We present two results in this direction. - Two-sided error in the high agreement regime. We begin with a brief remark about a reduction for high agreement algorithms, i.e., an algorithm which agrees with the correct output on a large (say > 0.9) fraction of entries, and show that the standard self-correction of linearity allows us to transform such algorithms into algorithms that work in worst case. - One-sided error in the low agreement regime. Focusing on average case algorithms with one-sided error, we show that over 𝔽₂ there is a reduction that gets an O(T) time average case algorithm that given a random input A,B outputs a matrix that agrees with A ⋅ B on at least 51% of the entries (i.e., has only a slight advantage over the trivial algorithm), and transforms it into an Õ(T) time worst case algorithm, that outputs the correct answer for all inputs with high probability.
@InProceedings{gola_et_al:LIPIcs.APPROX/RANDOM.2024.34, author = {Gola, Ashish and Shinkar, Igor and Singh, Harsimran}, title = {{Matrix Multiplication Reductions}}, booktitle = {Approximation, Randomization, and Combinatorial Optimization. Algorithms and Techniques (APPROX/RANDOM 2024)}, pages = {34:1--34:15}, series = {Leibniz International Proceedings in Informatics (LIPIcs)}, ISBN = {978-3-95977-348-5}, ISSN = {1868-8969}, year = {2024}, volume = {317}, editor = {Kumar, Amit and Ron-Zewi, Noga}, publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik}, address = {Dagstuhl, Germany}, URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.APPROX/RANDOM.2024.34}, URN = {urn:nbn:de:0030-drops-210274}, doi = {10.4230/LIPIcs.APPROX/RANDOM.2024.34}, annote = {Keywords: Matrix Multiplication, Reductions, Worst case to average case reductions} }
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