SOLVED

If $A\subseteq \mathbb{N}$ is a multiset of integers such that
\[\lvert A\cap \{1,\ldots,N\}\rvert\gg N\]
for all $N$ then must $A$ be subcomplete? That is, must
\[P(A) = \left\{\sum_{n\in B}n : B\subseteq A\textrm{ finite }\right\}\]
contain an infinite arithmetic progression?

A problem of Folkman. Folkman [Fo66] showed that this is true if
\[\lvert A\cap \{1,\ldots,N\}\rvert\gg N^{1+\epsilon}\]
for some $\epsilon>0$ and all $N$.

The original question was answered by Szemerédi and Vu [SzVu06] (who proved that the answer is yes).

This is best possible, since Folkman [Fo66] showed that for all $\epsilon>0$ there exists a multiset $A$ with \[\lvert A\cap \{1,\ldots,N\}\rvert\ll N^{1+\epsilon}\] for all $N$, such that $A$ is not subcomplete.