Euler Phi Function of Prime

Theorem
Let $p$ be a prime number $p > 1$.

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

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

Proof
From the definition of a prime number, the only (strictly) positive integer less than or equal to a prime $p$ which is not prime to $p$ is $p$ itself.

Thus it follows directly that:
 * $\phi \left({p}\right) = p - 1$