Dirichlet kernel
The
Dirichlet kernel
named after
Johann Peter Gustav Lejeune Dirichlet, is 2π times the
nth-degree
Fourier series approximation to a "function" with period 2π given by
-
where δ is the
Dirac delta function, which is not really a function, in the sense of mapping one set into another, but is rather a "generalized function", also called a "distribution". In other words, the Fourier series representation of this "function" is
-
This "periodic delta function" is the identity element for the
convolution defined on functions of period 2π by
-
In other words, we have
-
for every function
f of period 2π.
The convolution of
Dn(
x) with any function
f of period 2π is the
nth-degree Fourier series approximation to
f, i.e., we have
-
where
-
is the
kth Fourier coefficient of
f.
The trigonometric identity displayed at the top of this article may be established as follows. First recall that the sum of a finite geometric series is
-
The first term is
a; the common ratio by which each term is multiplied to get the next is
r; the number of terms is
n + 1. In particular, we have
-
The expression to the left of "=" should make us expect the sum to be a symmetric function of
r and 1/
r, but the expression to the right of "=" is perhaps less-than-obviously symmetric in those two quantities. The remedy is to multiply both the numerator and the denominator by
r-1/2, getting
-
In case
r =
eix we have
-
and then "-2
i" cancels.