Functional Derivative (Paperback)

High Quality Content by WIKIPEDIA articles! In mathematics and theoretical physics, the functional derivative is a generalization of the directional derivative. The difference is that the latter differentiates in the direction of a vector, while the former differentiates in the direction of a function. Both of these can be viewed as extensions of the usual calculus derivative. Two possible, restricted definitions suitable for certain computations are given here. There are more general definitions of functional derivatives relying on ideas from functional analysis, such as the Gateaux derivative.