Some A,B,C,D from div. 2-3, Educationals have a math nature. So if recognize the math essence, the solution will be written fast, because as I analyses archive these implementation are not hard enough.

That's why the question is what the math core for competitive programming and how to improve level in this one for completely newbie.

I think it's possible to divide the question to two parts. What are recommended literature (можно и на русском) and what resources are good for train exactly mathematics for CP and CF in particular.

Look forward to your advice.