Green's Open Problems #8

### What is the conjecture
https://people.maths.ox.ac.uk/greenbj/papers/open-problems.pdf#problem.8

Suppose that $A \subset [N]$ has no more than $\varepsilon N^2$ solutions to $x + y = z$. It is known that one may remove $\varepsilon' N$ elements of $A$ to leave a sum-free set, where $\varepsilon' \to 0$ as $\varepsilon \to 0$. But is there a bound for $\varepsilon '$ in terms of $\varepsilon$ which is at least vaguely reasonable?

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