Partial derivative - Wikipedia
https://en.wikipedia.org/wiki/Partial_derivativeSecond and higher order partial derivatives are defined analogously to the higher order derivatives of univariate functions. For the function the "own" second partial derivative with respect to x is simply the partial derivative of the partial derivative (both with respect to x): The cross partial derivative with respect to x and y is obtained by taking the partial derivative of f with respect to x, and then taking the partial derivative of the result with respect to y, to obtain