Is there any circulant $(-1,1)$-matrix satisfying the following property? Let $n$ be a positive integer greater than $13$, is there any n-by-n circulant $(-1,1)$-matrix $A$ satisfying the following property: $$AA^T=(n-1)I+J$$ where $I$ is the n-by-n identity matrix and $J$ is the n-by-n matrix of ones. I conjecture that the answer is no. But I can't prove it.