From Sylvester-Gallai Configurations to Rank Bounds: Improved Black-box Identity Test for Depth-3 Circuits Nitin Saxena C. Seshadhri Hausdorff Center for Mathematics IBM Almaden Bonn - 53115, Germany San Jose - 95126, USA
[email protected] [email protected] Abstract—We study the problem of identity testing for depth- A depth-3 circuitC over a field F is of the form 3 k d k circuits of top fanin and degree .Wegiveanew C(x1, ··· ,xn)= i=1 Ti, where Ti (a multiplication structure theorem for such identities. A direct application of O(k) term) is a product of at most d linear polynomials with dk our theorem improves the known deterministic -time coefficients in F. We are especially interested in the case black-box identity test over rationals (Kayal & Saraf, FOCS O k2 F = Q. In this section, we will just assume this unless 2009) to one that takes d ( )-time. Our structure theorem essentially says that the number of independent variables in a explicitly mentioned otherwise. The size of the circuit real depth-3 identity is very small. This theorem affirmatively C can be expressed in three parameters: the number settles the strong rank conjecture posed by Dvir & Shpilka of variables n, the degree d, and the top fanin (or the (STOC 2005). number of terms) k. Such a circuit is referred to as a We devise a powerful algebraic framework and develop tools ΣΠΣ(k, d, n) circuit. PIT algorithms for depth-3 circuits to study depth-3 identities. We use these tools to show that any depth-3 identity contains a much smaller nucleus identity were first studied by Dvir & Shpilka [DS06].