Hi everyone

I would like to know if you could recommend me some sites or repositories with math problems, I mean pure math problems,I've noticed that, math skills are very important for Competitive Programming, and most of math problems here have some computer trick, that I understand, but I usually don't get the background of the solution, even in Greedy Problems, frequently the solution have some background math, so I would like to train with Math Problems, Is there a site for that? I mean like codeforces, but for mathematicians.