Functional Derivative

In mathematics and theoretical physics, the functional derivative is a generalization of the usual derivative that arises in the calculus of variations. In a functional derivative, instead of differentiating a function with respect to a variable, one differentiates a functional with respect to a function. Two possible, restricted definitions suitable for certain computations are given here. There are more general definitions of functional derivatives. For any functional F mapping (continuous/smooth/with certain boundary conditions/etc.) functions φ from a manifold M to \mathbb{R} or \mathbb{C}, then, provided the following derivative exists, the functional derivative
\frac{\delta F}{\delta \phi}\phi
is a distribution such that for all test functions f,
\left(\frac{\delta F}{\delta
\phi}\phi\right)f=\frac{d}{d\epsilon}Ff. Another definition is in terms of a limit and the Dirac delta function, δ:
\frac{\delta F\phi(x)}{\delta \phi(y)}=\lim_{\varepsilon\to 0}\frac{F\phi(x)+\varepsilon\delta(x-y)-F\phi(y)}{\varepsilon}.

 

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