Euler Phi Function of Prime Power

Theorem
Let $p^n$ be a prime power for some prime number $p > 1$.

Then:
 * $\phi \left({p^n}\right) = p^n \left({1 - \dfrac 1 p}\right) = \left({p - 1}\right) p^{n - 1}$

where $\phi: \Z_{>0} \to \Z_{>0}$ is the Euler $\phi$ function.

Corollary
When $p = 2$, the formula is exceptionally simple:

Proof
From Euler Phi Function of Prime:
 * $\phi \left({p}\right) = p - 1$

From Prime not Divisor implies Coprime:
 * $k \perp p^n \iff p \nmid k$

There are $p^{n - 1}$ integers $k$ such that $1 \le k \le p^n$ which are divisible by $p$:


 * $k \in \left\{{p, 2 p, 3 p, \ldots, \left({p^{n - 1}}\right) p}\right\}$

Therefore:
 * $\phi \left({p^n}\right) = p^n - p^{n-1} = p^n \left({1 - \dfrac 1 p}\right) = \left({p - 1}\right) p^{n - 1}$

When $n = 1$, note that:
 * $\phi \left({p^1}\right) = p^1 \left({1 - \dfrac 1 p}\right) = p - 1$

demonstrating the compatibility of these definitions.

Also see

 * Euler Phi Function of Prime: When $n = 1$ the expression degenerates to $\phi \left({p}\right) = p - 1$.