On the Structure of Quintic Polynomials

We study the structure of bounded degree polynomials over finite fields. Haramaty and Shpilka [STOC 2010] showed that biased degree three or four polynomials admit a strong structural property. We confirm that this is the case for degree five polynomials also. Let $\mathbb{F}=\mathbb{F}_q$ be a prime field. [1.] Suppose $f:\mathbb{F}^n\rightarrow \mathbb{F}$ is a degree five polynomial with bias(f)=\delta. Then f can be written in the form $f= \sum_{i=1}^{c} G_i H_i + Q$, where $G_i$ and $H_i$s are nonconstant polynomials satisfying $deg(G_i)+deg(H_i)\leq 5$ and $Q$ is a degree $\leq 4$ polynomial. Moreover, $c=c(\delta)$ does not depend on $n$ and $q$. [2.] Suppose $f:\mathbb{F}^n\rightarrow \mathbb{F}$ is a degree five polynomial with $bias(f)=\delta$. Then there exists an $\Omega_\delta(n)$ dimensional affine subspace $V$ of $\mathbb{F}^n$ such that $f$ restricted to $V$ is a constant. Cohen and Tal [Random 2015] proved that biased polynomials of degree at most four are constant on a subspace of dimension $\Omega(n)$. Item [2.] extends this to degree five polynomials. A corollary to Item [2.] is that any degree five affine disperser for dimension $k$ is also an affine extractor for dimension $O(k)$. We note that Item [2.] cannot hold for degrees six or higher. We obtain our results for degree five polynomials as a special case of structure theorems that we prove for biased degree d polynomials when $d<|\mathbb{F}|+4$. While the $d<|\mathbb{F}|+4$ assumption seems very restrictive, we note that prior to our work such structure theorems were only known for $d<|\mathbb{F}|$ by Green and Tao [Contrib. Discrete Math. 2009] and Bhowmick and Lovett [arXiv:1506.02047]. Using algorithmic regularity lemmas for polynomials developed by Bhattacharyya, et. al. [SODA 2015], we show that whenever such a strong structure exists, it can be found algorithmically in time polynomial in n.