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TR14-154 | 20th November 2014 04:16

Fast Matrix Multiplication: Limitations of the Laser Method


Authors: Andris Ambainis, Yuval Filmus, Francois Le Gall
Publication: 20th November 2014 04:20
Downloads: 2075


Until a few years ago, the fastest known matrix multiplication algorithm, due to Coppersmith and Winograd (1990), ran in time $O(n^{2.3755})$. Recently, a surge of activity by Stothers, Vassilevska-Williams, and Le Gall has led to an improved algorithm running in time $O(n^{2.3729})$. These algorithms are obtained by analyzing higher and higher tensor powers of a certain identity of Coppersmith and Winograd. We show that this exact approach cannot result in an algorithm with running time $O(n^{2.3725})$, and identify a wide class of variants of this approach which cannot result in an algorithm with running time $O(n^{2.3078})$; in particular, this approach cannot prove the conjecture that for every $\epsilon > 0$, two $n\times n$ matrices can be multiplied in time $O(n^{2+\epsilon})$.
We describe a new framework extending the original laser method, which is the method underlying the previously mentioned algorithms. Our framework accommodates the algorithms by Coppersmith and Winograd, Stothers, Vassilevska-Williams and Le Gall. We obtain our main result by analyzing this framework. The framework is also the first to explain why taking tensor powers of the Coppersmith-Winograd identity results in faster algorithms.

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