What is the conjecture

https://people.maths.ox.ac.uk/greenbj/papers/open-problems.pdf#problem.82

Let $A \subset \mathbb{Z}$ be a set of size $n$. For how many $\theta \in \mathbb{R}/\mathbb{Z}$ must we have $\sum_{a \in A} \cos(a\theta) = 0$?

AMS categories

• ams-11
• ams-42

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• I plan on adding this conjecture to the repository
• This issue is up for grabs: I would like to see this conjecture added by somebody else