Factors of Sparse Polynomials are Sparse

This paper was removed due to an error in the proof (Claim 4.12 as stated is not true). The authors would like to thank Ilya Volkovich for pointing out a counterexample to this papers main result in positive characteristic: If $F$ is a field with prime characteristic $p$, then the polynomial $x_1^p + x_2^p + ldots + x^n^p$ has the following factor: $(x_1+x_2+ ldots + x_n)^{p-1}$, which has sparsity $n^p$.

Testing Equivalence of Polynomials under Shifts

Two polynomials $f, g in mathbb{F}[x_1, ldots, x_n]$ are called shift-equivalent if there exists a vector $(a_1, ldots, a_n) in mathbb{F}^n$ such that the polynomial identity $f(x_1+a_1, ldots, x_n+a_n) equiv g(x_1,ldots,x_n)$ holds. Our main result is a new randomized algorithm that tests whether two given polynomials are shift equivalent. Our algorithm runs in time polynomial in the circuit size of the polynomials, to which it is given black box access. This complements a previous work of Grigoriev (Theoretical Computer Science, 1997) who gave a deterministic algorithm running in time $n^{O(d)}$ for degree $d$ polynomials. Our algorithm uses randomness only to solve instances of the Polynomial Identity Testing (PIT) problem. Hence, if one could de-randomize PIT (a long-standing open problem in complexity) a de-randomization of our algorithm would follow. This establishes an equivalence between de-randomizing shift-equivalence testing and de-randomizing PIT (both in the black-box and the white-box setting). For certain restricted models, such as Read Once Branching Programs, we already obtain a deterministic algorithm using existing PIT results.