Revisiting sums and products in countable and finite fields

We establish a polynomial ergodic theorem for actions of the affine group of a countable field $K$. As an application, we deduce–via a variant of Furstenberg’s correspondence principle–that for fields of characteristic zero, any “large” set $E\subset K$ contains “many” patterns of the form $\{p(x)+y,xy\}$, for every non-constant polynomial $p(x)\in K[x]$. Our methods are flexible enough that they allow us to recover analogous density results in the setting of finite fields and, with the aid of a new finitistic variant of Bergelson’s “colouring trick”, show that for $r\in \mathbb{N}$ fixed, any $r-$colouring of a large enough finite field will contain monochromatic patterns of the form $\{x,p(x)+y,xy\}$. In a different direction, we obtain a double ergodic theorem for actions of the affine group of a countable field. An adaptation of the argument for affine actions of finite fields leads to a generalisation of a theorem of Shkredov. Finally, to highlight the utility of the aforementioned finitistic “colouring trick”, we provide a conditional, elementary generalisation of Green and Sanders’ $\{x,y,x+y,xy\}$ theorem.