Polynomial corners in finite fields beyond the distinct-degree case

Abstract

We prove a quantitative polynomial Roth theorem for corners in \(\mathbb F_p^2\) for arbitrary pairs of linearly independent polynomials. More precisely, given a positive integer $d$, there are constants $p_0$ and $C$ (depending only on $p$) so that for every $p>p_0$, if polynomials \(ϕ_1,ϕ_2\in \mathbb \mathbb{F}_p [y]\) are of degree $\le d$ vanishing at $0$ and are not linearly dependent, then every \(A\subset\mathbb F_p^2\) with $|A|\ge C{p}^{2-1/14}$ contains a nontrivial corner $$(x_1,x_2),(x_1+{\varphi }_1(y),x_2),(x_1,x_2+{\varphi }_2(y))$$ for some \(y\in\mathbb F_p^\times\). This improves the estimate $p^{2-1/16}$ of Han--Lacey--Yang and removes the distinct-degree restriction from their quantitative theorem. The main obstruction is the equal-degree resonant case, where the Jacobian argument of Han--Lacey--Yang degenerates. We adjoin the frequency-independent part of the phase to form an augmented map \(\widetilde F:W\to\mathbb A^3\) from the correlation threefold. We prove that this map is generically finite on every top-dimensional geometric component and has no two-dimensional fibre. Using the associated Artin--Schreier sheaf and Katz--Laumon estimates for Fourier transform of perverse sheaves, we obtain square-root cancellation outside an algebraic exceptional set of dimension at most one and uniformly bounded degree. A separate curve-sum argument gives uniform control on the exceptional set. An \(\ell^2\) matrix estimate adapted to such sets completes the resonant case.