Green's Open Problems #90

### What is the conjecture
https://people.maths.ox.ac.uk/greenbj/papers/open-problems.pdf#problem.90

Suppose that $A \subset \mathbb{Z}/p\mathbb{Z}$ is a set of density $\frac{1}{2}$. Under what conditions on $K$ can $A$ be almost invariant under all maps $\phi(x) = ax + b$, $|a|, |b| \leq K$?

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

* 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

Start with the linked Green paper, specifically problem 90, to understand the conjecture and its quantifiers. Then inspect the repository's existing formalized conjecture entries for the appropriate way to add this statement. Done means the conjecture is added to the collection in a reviewed, type-correct form.

Written by the indexing model from the issue text.