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Let $A\subseteq \mathbb{F}_p$. Let \[A\hat{+}A = \{ a+b : a\neq b \in A\}.\] Is it true that \[\lvert A\hat{+}A\rvert \geq \min(2\lvert A\rvert-3,p)?\]
A question of Erdős and Heilbronn. Solved in the affirmative by da Silva and Hamidoune [dSHa94].