A sparse polynomial (also called a lacunary polynomial) is a polynomial that
has relatively few terms compared to its degree. The sparse-representation of a
polynomial represents the polynomial as a list of its non-zero terms
(coefficient-degree pairs). In particular, the degree of a sparse polynomial
can be exponential in the sparse-representation size.
We prove that for monic polynomials f,gβC[x] such that g
divides f, the β2β-norm of the quotient polynomial f/g is bounded by
β₯fβ₯1ββ O~(β₯gβ₯03βdeg2f)β₯gβ₯0ββ1. This improves upon the exponential (in
degf) bounds for general polynomials and implies that the trivial
long division algorithm runs in time quasi-linear in the input size and number
of terms of the quotient polynomial f/g, thus solving a long-standing problem
on exact divisibility of sparse polynomials.
We also study the problem of bounding the number of terms of f/g in some
special cases. When f,gβZ[x] and g is a cyclotomic-free
(i.e., it has no cyclotomic factors) trinomial, we prove that
β₯f/gβ₯0ββ€O(β₯fβ₯0βsize(f)2β log6degg). When g is a binomial with g(Β±1)ξ =0, we
prove that the sparsity is at most O(β₯fβ₯0β(logβ₯fβ₯0β+logβ₯fβ₯ββ)). Both upper bounds
are polynomial in the input-size. We leverage these results and give a
polynomial time algorithm for deciding whether a cyclotomic-free trinomial
divides a sparse polynomial over the integers.
As our last result, we present a polynomial time algorithm for testing
divisibility by pentanomials over small finite fields when degf=O~(degg)