Query-efficient algorithms for polynomial interpolation over composites

  • Authors:
  • Parikshit Gopalan

  • Affiliations:
  • Georgia Tech., Atlanta Ga

  • Venue:
  • SODA '06 Proceedings of the seventeenth annual ACM-SIAM symposium on Discrete algorithm
  • Year:
  • 2006

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Abstract

The problem of polynomial interpolation is to reconstruct a polynomial based on its valuations on a set of inputs I. We consider the problem over Zm when m is composite. We ask the question: Given I ⊆ Zm, how many evaluations of a polynomial at points in I are required to compute its value at every point in I? Surprisingly for composite m, this number can vary exponentially between log[I] and [I] in contrast to the prime case where [I] evaluations are necessary. While this minimization problem is NP-complete, we give an efficient algorithm of query complexity within a factor t of the optimum where t is the number of prime factors of m. We use our interpolation algorithm to design algorithms for zero-testing and distributional learning of polynomials over Zm. In some cases, we get an exponential improvement over known algorithms in query complexity and running time. Our main technical contribution is the notion of an interpolating set for I which is a subset S of I such that a polynomial which is 0 over S must be 0 at every point in I. Any interpolation algorithm needs to query an interpolating set for I. Our query-efficient algorithms are obtained by constructing interpolating sets whose size is close to optimal.