# Dirichlet kernel

In mathematical analysis, the **Dirichlet kernel** is the collection of functions

It is named after Peter Gustav Lejeune Dirichlet.

The importance of the Dirichlet kernel comes from its relation to Fourier series. The convolution of *D _{n}*(

*x*) with any function

*f*of period 2π is the

*n*th-degree Fourier series approximation to

*f*, i.e., we have

where

is the *k*th Fourier coefficient of *f*. This implies that in order to study convergence of Fourier series it is enough to study properties of the Dirichlet kernel. Of particular importance is the fact that the *L*^{1} norm of *D _{n}* diverges to infinity as

*n*→ ∞. One can estimate that

By using a Riemann-sum argument to estimate the contribute in the largest neighbourhood of zero in which is positive, and the Jensen's inequality for the remaining part, it is also possible to show that:

This lack of uniform integrability is behind many divergence phenomena for the Fourier series. For example, together with the uniform boundedness principle, it can be used to show that the Fourier series of a continuous function may fail to converge pointwise, in rather dramatic fashion. See convergence of Fourier series for further details.

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## Relation to the delta function

Take the periodic 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", and multiply by 2π. We get the identity element for convolution on functions of period 2π. In other words, we have

for every function *f* of period 2π. The Fourier series representation of this "function" is

Therefore the Dirichlet kernel, which is just the sequence of partial sums of this series, can be thought of as an *approximate identity*. Abstractly speaking it is not however an approximate identity of *positive* elements (hence the failures mentioned above).

## Proof of 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

In particular, we have

Multiply both the numerator and the denominator by *r*^{−1/2}, getting

In the case *r* = *e*^{ix} we have

as required.

### Alternative proof of the trigonometric identity

Start with the series

Multiply both sides of the above by

and use the trigonometric identity

to reduce the r.h.s. to

## Variant of identity

If the sum is only over positive integers (which may arise when computing a DFT that is not centered), then using similar techniques we can show the following identity:

## See also

## References

- Andrew M. Bruckner, Judith B. Bruckner, Brian S. Thomson:
*Real Analysis*. ClassicalRealAnalysis.com 1996, ISBN 0-13-458886-X, S.620 (vollständige Online-Version (Google Books)) - Podkorytov, A. N. . (1988), "Asymptotic behavior of the Dirichlet kernel of Fourier sums with respect to a polygon".
*Journal of Soviet Mathematics*, 42(2): 1640–1646. doi: 10.1007/BF01665052 - Levi, H. . (1974), "A geometric construction of the Dirichlet kernel".
*Transactions of the New York Academy of Sciences*, 36: 640–643. doi: 10.1111/j.2164-0947.1974.tb03023.x - {{#invoke:citation/CS1|citation

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