ECCC-Report TR08-108https://eccc.weizmann.ac.il/report/2008/108Comments and Revisions published for TR08-108en-usThu, 11 Dec 2008 19:25:55 +0200
Paper TR08-108
| An Almost Optimal Rank Bound for Depth-3 Identities |
Nitin Saxena,
C. Seshadhri
https://eccc.weizmann.ac.il/report/2008/108We 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.
Thu, 11 Dec 2008 19:25:55 +0200https://eccc.weizmann.ac.il/report/2008/108