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We prove that certain instances of the iterated matrix multiplication (IMM) family of polynomials with N variables and degree n require N^(Omega(sqrt(n))) gates when expressed as a homogeneous depth-five Sigma Pi Sigma Pi Sigma arithmetic circuit with the bottom fan-in bounded by N^(1/2-epsilon). By a depth-reduction result of Tavenas, this size lower bound is optimal and can be achieved by the weaker class of homogeneous depth-four Sigma Pi Sigma Pi circuits. Our result extends a recent result of Kumar and Saraf, who gave the same N^(Omega(sqrt(n))) lower bound for homogeneous depth-four Sigma Pi Sigma Pi circuits computing IMM. It is analogous to a recent result of Kayal and Saha, who gave the same lower bound for homogeneous Sigma Pi Sigma Pi Sigma circuits (over characteristic zero) with bottom fan-in at most N^(1-epsilon), for the harder problem of computing certain polynomials defined by Nisan-Wigderson designs.
@InProceedings{bera_et_al:LIPIcs.CCC.2015.183,
author = {Bera, Suman K. and Chakrabarti, Amit},
title = {{A Depth-Five Lower Bound for Iterated Matrix Multiplication}},
booktitle = {30th Conference on Computational Complexity (CCC 2015)},
pages = {183--197},
series = {Leibniz International Proceedings in Informatics (LIPIcs)},
ISBN = {978-3-939897-81-1},
ISSN = {1868-8969},
year = {2015},
volume = {33},
editor = {Zuckerman, David},
publisher = {Schloss Dagstuhl -- Leibniz-Zentrum f{\"u}r Informatik},
address = {Dagstuhl, Germany},
URL = {https://drops.dagstuhl.de/entities/document/10.4230/LIPIcs.CCC.2015.183},
URN = {urn:nbn:de:0030-drops-50622},
doi = {10.4230/LIPIcs.CCC.2015.183},
annote = {Keywords: arithmetic circuits, iterated matrix multiplication, depth five circuits, lower bound}
}