We have two natural numbers, m and n, and m divides n.
Prove, that $\phi(m)$ divides φ(n) too for every m and n.
φ(a) means the Euler's totient function: for an a positive integers, it is the number of lower numbers which are relative prime to a.
For example, φ(4)=2, because only two numbers, 1, and 3 are relative prime to 4.
This question may be a duplicate, but I didn't find any of it yet, if so, I apologise in advance.