Green's Open Problems #48

### What is the conjecture
https://people.maths.ox.ac.uk/greenbj/papers/open-problems.pdf#problem.48

Suppose that a small sieve process leaves a set of maximal size. What is the structure of that set?

### [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

Read problem 48 in the linked Green's Open Problems PDF and inspect the repository's existing formalized conjectures for the expected structure. Determine the precise mathematical statement and what formalization is needed; done means the conjecture is added to the repository in the established format and its checks pass.

Written by the indexing model from the issue text.