Main Question or Discussion Point
Prove that if d divides n then phi(d) divides phi(n).
Learn what mulitplicative means.I'm making a quick comment
Euler's totient function IS multiplicative. Someone said it's only for coprimes but there's a general form where the 2 numbers dont have to be coprime
phi(mn) = phi(m)phi(n) * d/phi(d)
where d is the GCD of m and n.