An asymmetric version of Elekes-Szabó via group actions

Abstract

We consider when finite families $F\subseteq \mathbb{C}[t]$ of bounded degree polynomials, or more generally of bounded complexity finite-to-finite correspondences on $\mathbb{C}$, can exhibit non-expansion of the form $|F(A)|=O(|A{|}^{1+\eta })$ in their actions on finite sets $A\subseteq \mathbb{C}$ with $|F| \gg |A|^\eps \gg 1$, for a fixed $\eps>0$ and arbitrarily small $\eta >0$. Our conclusions generalise the Elekes-Rónyai and Elekes-Szabó theorems, which correspond to the case that $F$ is parametrised by a single complex variable and $|F|=|A|$. Our result also applies to families of correspondences between varieties of arbitrary dimension if we impose a general position assumption on $A$. In all cases, the conclusion is that a commutative algebraic group structure is responsible. As a special case, we obtain asymmetric versions of Elekes-Rónyai and Elekes-Szabó, with explicit bounds on exponents. Our methods originate in model theory.

v2: Obtain bounds on exponents in some of the results. Add application to multivariate unbalanced Elekes-Rónyai; v3: update references; v4: extensive reworking in many sections -- numbering changed, main results unchanged