Some numerical libraries I'm gone to use in a near-future to make my work easier.
Simple and fast and improvable as possible code to calculate the Euler's Phi function (all of coprimes number between 0 and n) of a unsigned number, and uses a vector to store all prime dividers of the n number to use with another functions.
It uses more interesting functions, like a Eratosthenes Sieve, to keep this working fast.