Euler Phi Function of 5

Theorem

$\map \phi 5 = 4$

where $\phi$ denotes the Euler $\phi$ function.

Proof

From Euler Phi Function of Prime:

$\map \phi p = p - 1$

As $5$ is a prime number it follows that:

$\map \phi 5 = 5 - 1 = 4$

$\blacksquare$