The Erdos-Turan problem in infinite groups
Let $G$ be an infinite abelian group with $|2G|=|G|$. We show that if $G$ is not the direct sum of a group of exponent 3 and the group of order 2, then $G$ possesses a perfect additive basis; that is, there is a subset $S\subseteq G$ such that every element of $G$ is uniquely representable as a sum of two elements of $S$. Moreover, if $G$ \emph{is} the direct sum of a group of exponent 3 and the group of order 2, then it does not have a perfect additive basis; however, in this case there is a subset $S\subseteq G$ such that every element of $G$ has at most two representations (distinct under permuting the summands) as a sum of two elements of $S$. This solves completely the Erdos-Turan problem for infinite groups. It is also shown that if $G$ is an abelian group of exponent 2, then there is a subset $S\subseteq G$ such that every element of $G$ has a representation as a sum of two elements of $S$, and the number of representations of non-zero elements is bounded by an absolute constant.