Hessian matrix

A Hessian matrix is "a square matrix of second-order partial derivatives of a scalar-valued function, or scalar field."

$$H = \begin{pmatrix}\frac{\delta^2 f}{\delta x^2} & \frac{\delta^2 f}{\delta x \delta y} \\ \frac{\delta^2 f}{\delta y \delta x} & \frac{\delta^2 f}{\delta y^2}\end{pmatrix}$$

Another way to write the same equation is

$$H = \begin{pmatrix}\frac{\delta^2 f}{\delta xx} & \frac{\delta^2 f}{\delta xy} \\ \frac{\delta^2 f}{\delta yx} & \frac{\delta^2 f}{\delta yy}\end{pmatrix}$$