Hi
I'm preparing my CV and on top of it I want to write a short description of myself, that just says my area of "expertise" and what job I'm looking for.
Would it sound stupid to have something like this:
"Specialist in mathematics"
i.e. is it unscientific or something to call oneself a "specialist"?

It depends on who the CV is aimed for. If it's a general CV for a non-academic post I think that sounds ok but for an academic post I think there needs to a bit more detail on which area you specialise in and what research you have done in this area.