Functional derivative (source code)

= Functional derivative
{wiki=Functional_derivative}

The concept of a functional derivative is a generalization of the ordinary derivative to functionals, which are mappings from a space of functions to the real numbers (or complex numbers). In essence, while a regular derivative gives the rate of change of a function with respect to its variables, a functional derivative captures the rate of change of a functional with respect to changes in the function it depends on.