Erdős problem 328

Suppose $A\subseteq\mathbb{N}$ and $C>0$ is such that $1_A\ast 1_A(n)\leq C$ for all $n\in\mathbb{N}$. Can $A$ be partitioned into $t$ many subsets $A_1,\ldots,A_t$ (where $t=t(C)$ depends only on $C$) such that $1_{A_i}\ast 1_{A_i}(n)<C$ for all $1\leq i\leq t$ and $n\in \mathbb{N}$?

The answer is no. Asked by Erdős and Newman. Nešetřil and Rödl [NeRo85] have shown the answer is no for all $C$ (even if $t$ is also allowed to depend on $A$).

Erdős [Er80e] had previously shown the answer is no for $C=3,4$ and infinitely many other values of $C$.

See also [774].

The linked proof writes the representation function as Set.ncard {p : ℕ × ℕ | p.1 ∈ A ∧ p.2 ∈ A ∧ p.1 + p.2 = n}, which counts the same ordered pairs as sumRep, and states the partition condition as a named definition with the same two conjuncts used below. Its ∃ t additionally carries 1 ≤ t, which costs nothing: t = 0 forces A = ∅, and A = {1} has all representation counts at most C for C ≥ 1.