If I have a Hessian matrix $ \begin{bmatrix} 0 & 1 \\ 1 & 0 \end{bmatrix} $ might someone help me understand why this is not positive semidefinite?
My understanding that if for any $q(x) - x^TAx$, $A$ is PSD if $q(x) \le 0$ for every $x \in R^n$.
2026-03-30 14:55:43.1774882543
Positive semidefinite matrix with 0 on diagonal
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