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Algorithms for modular counting of roots of multivariate polynomials. (English) Zbl 1143.11046

Summary: Given a multivariate polynomial \(P(X_{1},\ldots, X_{n})\) over a finite field \({\mathbb {F}_{q}}\), let \(N (P)\) denote the number of roots over \({\mathbb {F}_{q}^{n}}\). The modular root counting problem is, given a modulus \(r\), to determine \(N_{r}(P)= N(P)\mod r\). We study the complexity of computing \(N_{r}(P)\), when the polynomial is given as a sum of monomials. We give an efficient algorithm to compute \(N_{r}(P)\) when the modulus \(r\) is a power of the characteristic of the field. We show that for all other moduli, the problem of computing \(N_{r}(P)\) is NP-hard. We present some hardness results which imply 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.

MSC:

11Y16 Number-theoretic algorithms; complexity
11T06 Polynomials over finite fields
11T71 Algebraic coding theory; cryptography (number-theoretic aspects)
68Q17 Computational difficulty of problems (lower bounds, completeness, difficulty of approximation, etc.)
68W30 Symbolic computation and algebraic computation
94B35 Decoding
Full Text: DOI

References:

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