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.
you can try these ...
FIRST
SECOND
You can check it too..... Third
NICE INFO!
There are many material on Maths. I can share some that I know.
Maths problems in Div2 A, B or C are largely dominated by
constructive,invariant-finding,number-theory,inductionetc.Reading Materials:
twochapters from this book and this the best thing I have read till now.Reading the solutions after trying hard are quite effective.Practice Materials :
resources 1,2 and 3 that you listed are really awesome. I remember them from MO's.
Of course some basic number theory and combinatorics are necessary, but more important than any specific math topic is mathematical fluency and maturity in general. How good are you with proving? How good is your number sense? How good are you at making insights and observations?
It's not Project Euler over here. Most Div2-3 problems are solvable without much theory. Even a lot of more difficult items only need a handful of standard algorithms. If I were to compose a list of this "standard stuff" off the top of my head, I would mention