Distributional Derivatives
The distributional derivative of the Dirac delta distribution is the distribution δ′ defined on compactly supported smooth test functions φ by
The first equality here is a kind of integration by parts, for if δ were a true function then
The k-th derivative of δ is defined similarly as the distribution given on test functions by
In particular δ is an infinitely differentiable distribution.
The first derivative of the delta function is the distributional limit of the difference quotients:
More properly, one has
where τh is the translation operator, defined on functions by τhφ(x) = φ(x+h), and on a distribution S by
In the theory of electromagnetism, the first derivative of the delta function represents a point magnetic dipole situated at the origin. Accordingly, it is referred to as a dipole or the doublet function.
The derivative of the delta function satisfies a number of basic properties, including:
Furthermore, the convolution of δ' with a compactly supported smooth function ƒ is
which follows from the properties of the distributional derivative of a convolution.
Read more about this topic: Dirac Delta Function