From sylvester-gallai configurations to rank bounds: Improved blackbox identity test for depth-3 circuits

  • Authors:
  • Nitin Saxena;C. Seshadhri

  • Affiliations:
  • Hausdorff Center for Mathematics, Bonn, Germany;IBM Research, Almaden

  • Venue:
  • Journal of the ACM (JACM)
  • Year:
  • 2013

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Abstract

We study the problem of identity testing for depth-3 circuits of top fanin k and degree d. We give a new structure theorem for such identities that improves the known deterministic dkO(k)-time blackbox identity test over rationals [Kayal and Saraf, 2009] to one that takes dO(k2)-time. Our structure theorem essentially says that the number of independent variables in a real depth-3 identity is very small. This theorem affirmatively settles the strong rank conjecture posed by Dvir and Shpilka [2006]. We devise various algebraic tools to study depth-3 identities, and use these tools to show that any depth-3 identity contains a much smaller nucleus identity that contains most of the “complexity” of the main identity. The special properties of this nucleus allow us to get near optimal rank bounds for depth-3 identities. The most important aspect of this work is relating a field-dependent quantity, the Sylvester-Gallai rank bound, to the rank of depth-3 identities. We also prove a high-dimensional Sylvester-Gallai theorem for all fields, and get a general depth-3 identity rank bound (slightly improving previous bounds).