On three-variable expanders over finite valuation rings (2007.05251v2)

Abstract: Let $\mathcal{R}$ be a finite valuation ring of order $q^r$. In this paper, we prove that for any quadratic polynomial $f(x,y,z) \in \mathcal{R}[x,y,z]$ that is of the form $axy+R(x)+S(y)+T(z)$ for some one-variable polynomials $R, S , T$, we have [ |f(A,B,C)| \gg \min\left{ q^r, \frac{|A||B||C|}{q^{2r-1}}\right}] for any $A, B, C \subset \mathcal{R}$. We also study the sum-product type problems over finite valuation ring $\mathcal{R}.$ More precisely, we show that for any $A \subset \mathcal{R}$ with $|A| \gg q^{r-1/3}$ then $$\max{ |A \cdot A|, |A^d + A^d|},\max{ |A + A|, |A^2 + A^2|},\max{|A-A|,|AA+AA|} \gg |A|^{2/3}q^{r/3},$$ and $|f(A) + A| \gg |A|^{2/3}q^{r/3}$ for any one variable quadratic polynomial $f$.