Science Fair Project Encyclopedia
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.
Another definition is in terms of a limit and the Dirac delta function, δ:
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