Let $U\subset\mathbb R^n$ be an open subset and let $f:U\to\mathbb R$ be of class at least $C^2$. The Hessian matrix of $f$ at the point $a \in U$ is the matrix \(H = \begin{bmatrix} \frac{\partial^2 f}{\partial x_i\partial x_j} \end{bmatrix}_{i,j=1}^n\) of the mixed second partial derivatives of $f$.
Wikidata ID: Q620495