Green's Open Problems #30

### What is the conjecture
https://people.maths.ox.ac.uk/greenbj/papers/open-problems.pdf#problem.30

Given a set $A \subset \mathbb{Z}$ with $D(A) \leq K$, find a large structured subset $A′$ which ‘obviously’ has $D(A′) \leq K + \varepsilon$ (Here, $D(A) := |A - A|/|A|$).

### [AMS categories](https://github.com/google-deepmind/formal-conjectures/labels?q=ams-)

* ams-05
* ams-11

### 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

Read problem 30 in the linked Green Open Problems PDF, then inspect the formal-conjectures repository for its Lean conventions and nearby formalized entries. The work is done when this conjecture is represented in Lean and the repository accepts the addition.

Written by the indexing model from the issue text.