Euler Phi Function of 4

Example of Use of Euler $\phi$ Function

 * $\phi \left({4}\right) = 2$

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

Proof
From Euler Phi Function of Prime Power: Corollary:
 * $\phi \left({2^k}\right) = 2^{k - 1}$

Thus:
 * $\phi \left({4}\right) = \phi \left({2^2}\right) = 2^1 = 2$

They can be enumerated as:
 * $1, 3$