Infinite unrestricted sumsets in subsets of abelian groups with large density (2504.08649v1)

Abstract: Let $(G,+)$ be a countable abelian group such that the subgroup ${g+g\colon g\in G}$ has finite index and the doubling map $g\mapsto g+g$ has finite kernel. We establish lower bounds on the upper density of a set $A\subset G$ with respect to an appropriate F{\o}lner sequence, so that $A$ contains a sumset of the form ${t+b_1+b_2\colon b_1,b_2\in B}$ or ${b_1+b_2\colon b_1,b_2\in B}$, for some infinite $B\subset G$ and some $t\in G$. Both assumptions on $G$ are necessary for our results to be true. We also characterize the F{\o}lner sequences for which this is possible. Finally, we show that our lower bounds are optimal in a strong sense.