Erdős Problem 864

### What is the conjecture

https://www.erdosproblems.com/864

Let $A\subseteq \\{1,\ldots N\\}$ be a set such that there exists at most one $n$ with more than one solution to $n=a+b$ (with $a\leq b\in A$). Estimate the maximal possible size of $\lvert A\rvert$ - in particular, is it true that
$$\lvert A\rvert \leq (1+o(1))\frac{2}{\sqrt{3}}N^{1/2}?$$

Status: open

### Choose either option
- [ ] I plan on working on this conjecture
- [x] This issue is up for grabs: I would like to see this conjecture added by somebody else

Begin with the conjecture at https://www.erdosproblems.com/864 and inspect existing formalized conjectures in the repository to learn its expected conventions. The issue is done when Problem 864 has been added in the project's formalization style; no file or test is named here.

Written by the indexing model from the issue text.