On exact division and divisibility testing for sparse polynomials

Abstract

Assessing that a sparse polynomial G divides another sparse polynomial F is not yet known to admit a polynomial time algorithm. While computing the quotient Q = F quo G can be done in polynomial time with respect to the sparsities of F, G and Q, it is not yet sufficient to get a polynomial time divisibility test in general. Indeed, the sparsity of the quotient Q can be exponentially larger than the ones of F and G. In the favorable case where the sparsity #Q of the quotient is polynomial, the best known algorithm to compute Q has a non-linear factor #G#Q in the complexity, which is not optimal.In this work, we are interested in the two aspects of this problem. First, we propose a new randomized algorithm that computes the quotient of two sparse polynomials when the division is exact. Its complexity is quasi-linear in the sparsities of F , G and Q. Our approach relies on sparse interpolation and it works over any finite fields or the ring of integers. Then, as a step toward faster divisibility testing, we provide a new polynomial time algorithm when the divisor has some specific shapes. More precisely, we reduce the problem to finding a polynomial S such that QS is sparse and testing divisibility by S can be done in polynomial time. We identify some structure patterns in the divisor G for which we can efficiently compute such a polynomial S.