Hi there. I have this interesting problem which I don't know how to solve. I'll post it here because I think more people will se it, but I'm not sure if this is the proper subforum.

The problem says: How can be sure that isn't the Fourier series of a derivable function?

I thought that it doesn't accomplish the Diritchlet postulates, but it actually doesn't mean that it isn't a fourier series.

Does anyone know how to solve this?

Bye there and thanks.