Algorithms for modular counting of roots of multivariate polynomials

  • Authors:
  • Parikshit Gopalan;Venkatesan Guruswami;Richard J. Lipton

  • Affiliations:
  • College of Computing, Georgia Tech, Atlanta, GA;Department of Computer Science & Engineering, University of Washington, Seattle, WA;College of Computing, Georgia Tech, Atlanta, GA

  • Venue:
  • LATIN'06 Proceedings of the 7th Latin American conference on Theoretical Informatics
  • Year:
  • 2006

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Abstract

Given a multivariate polynomial P(X1, ⋯, Xn) over a finite field ${\mathbb F}_{q}$, let N(P) denote the number of roots over ${\mathbb F}^{n}_{q}$. The modular root counting problem is given a modulus r, to determine Nr(P) = N(P) mod r. We study the complexity of computing Nr(P), when the polynomial is given as a sum of monomials. We give an efficient algorithm to compute Nr(P) when the modulus r is a power of the characteristic of the field. We show that for all other moduli, the problem of computing Nr(P) is Nr(P) is NP-hard. We present some hardness results which imply that that our algorithm is essentially optimal for prime fields. We show an equivalence between maximum-likelihood decoding for Reed-Solomon codes and a root-finding problem for symmetric polynomials.