Chicago Journal of Theoretical Computer Science

Volume 2018

Article 5

Published by the Department of Computer Science, The University of Chicago.

The border support rank of two-by-two matrix multiplication is seven

Markus Bläser
Saarland University
mblaeser AT cs DOT uni-saarland DOT de

Matthias Christandl
University of Copenhagen
Copenhagen, Denmark
christandl AT math DOT ku DOT dk


Jeroen Zuiddam
CWI Amsterdam
University of Amsterdam
Amsterdam, Netherlands
j DOT zuidamm AT cwi DOT nl

August 25, 2018


We show that the border support rank of the tensor corresponding to two-by-two matrix multiplication is seven over the complex numbers. We do this by constructing two polynomials that vanish on all complex tensors with format four-by-four-by-four and border rank at most six, but that do not vanish simultaneously on any tensor with the same support as the two-by-two matrix multiplication tensor. This extends the work of Hauenstein, Ikenmeyer, and Landsberg. We also give two proofs that the support rank of the two-by-two matrix multiplication tensor is seven over any field: one proof using a result of De Groote saying that the decomposition of this tensor is unique up to sandwiching, and another proof via the substitution method. These results answer a question asked by Cohn and Umans. Studying the border support rank of the matrix multiplication tensor is relevant for the design of matrix multiplication algorithms, because upper bounds on the border support rank of the matrix multiplication tensor lead to upper bounds on the computational complexity of matrix multiplication, via a construction of Cohn and Umans. Moreover, support rank may be interpreted as a quantum communication complexity measure.

Submitted June 14, 2017, revised September 25, 2017, and August 21, 2018, published August 28, 2018.

DOI: 10.4086/cjtcs.2018.005

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