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TR08-108 | 19th November 2008 00:00

An Almost Optimal Rank Bound for Depth-3 Identities


Authors: Nitin Saxena, C. Seshadhri
Publication: 11th December 2008 19:25
Downloads: 1925


We show that the rank of a depth-3 circuit (over any field) that is simple,
minimal and zero is at most O(k^3\log d). The previous best rank bound known was
2^{O(k^2)}(\log d)^{k-2} by Dvir and Shpilka (STOC 2005).
This almost resolves the rank question first posed by Dvir and Shpilka
(as we also provide a simple and minimal identity of rank \Omega(k\log d)).

Our rank bound significantly improves (dependence on k exponentially reduced)
the best known deterministic black-box identity tests for depth-3 circuits
by Karnin and Shpilka (CCC 2008). Our techniques also shed light
on the factorization pattern of nonzero depth-3 circuits, most
strikingly: the rank of linear factors of a simple, minimal and nonzero depth-3 circuit
(over any field) is at most O(k^3\log d).

The novel feature of this work is a new notion of maps between sets
of linear forms, called ``ideal
matchings'', used to study depth-3 circuits. We prove interesting structural results about
depth-3 identities using these techniques. We believe that these can lead
to the goal of a deterministic polynomial time identity test for these circuits.

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