In this post I want to make a connection between Fourier coefficients and eigenvalues/spectrum. Let me put the claim up front:

**If , then is not invertible with respect to the convolution product.**

Please feel free to jump directly to the end for the explanation.

Let be identified with the circle . An integrable periodic function can be associated with its decomposition as a Fourier series

where the Fourier coefficients are defined by

Intuitively the Fourier coefficients tell us the weights of the components within f. is the spectrum of the signal .

In linear algebra, the spectrum of a square matrix (or more generally a bounded linear operator) is the set of eigenvalues is not invertible . Extending this, the spectrum of an element in a unital Banach algebra $A$ is is not invertible . Is there any way to connect this spectrum with the spectrum of a function?

Let us recall the definition of convolution of two functions:

It has the following property: . Note that is an algebra with respect to this convolution product (not with respect to the usual multiplication)! However, does not have an identity element (Is there a function such that f * g = f for all f?), we cannot talk about invertibility in this algebra. (Hence in harmonic analysis we need something called approximate identity as a substitute, sometimes appearing as summability kernel in classical case.) For convenience, let us move to a larger algebra which contains .

is the algebra of regular Borel measures on . A function is identified with the measure . The convolution product and Fourier coefficients of a measure is extended as follows:

and

is satisfied analogously.

Now we have an identity element in , namely the Dirac measure :

Its Fourier transform is for all .

Finally we can show our claim, by considering the contrapositive of the statement. Suppose is invertible, then

for some . Taking Fourier transform we have

for all . This implies that for all .

**Remark**:

1. Originally I want to prove the converse as well, but it involves some theory of Gelfand transform.

2. To be frank, this may not be the best way of presenting as eigenvalues. A more proper explanation would be along the line of eigenspace decomposition of the regular representation

In this case we can really claim that if and only if is an eigenvalue of .

3. I hope this post is still an interesting observation though, and will arouse your interest in the linkage between Fourier analysis, spectral theorem and representation theory. The interplay of the group action with the differential operator is behind the scene. One may also ponder on this question: why is so special?

Personally the motivation along this investigation is the line of different generalizations of spectrum, from eigenvalues to spectrum of a ring and a -algebra!