Let $\Phi_n(x)$ be the usual cyclotomic polynomial (minimal polynomial over the rationals for a primitive nth root of unity).
There are many well-known properties, such as $x^n-1 = \Pi_{d|n}\Phi_d(x)$.
The following fact appears to follow pretty easily:
Fact:
$\Phi_n(1)=p$ if $n$ is a prime power $p^k$.
$\Phi_n(1)=1$ if $n$ is divisible by more than one prime.
My question is, is there a reference for this fact? Or is it simple enough to just call it "folklore" or to just say it "follows easily from properties of cyclotomic polynomials".