[Off Topic] Suggest any Mailing Lists or Discussion Board for Mathematics?

I am reading the /ordinary differential equations /by V.I. Arnold. Now, 
I encountered some difficulties to understand some paragraphs. After 
examining what's the difficulties are, I consider the difficulties 
should originate from the lacking of some background knowledge or 
concepts. The problem is that I have no idea to choose any suitable 
stuff to compensate the lack.

Thanks in Advance.