Euler Phi Function/Examples/9

Example of Use of Euler $\phi$ Function
The value of the Euler $\phi$ function for the integer $9$ is $6$.

Proof
From Euler Phi Function of Prime Power:
 * $\phi \left({3^k}\right) = 2 \times 3^{k - 1}$

Thus:
 * $\phi \left({9}\right) = \phi \left({3^2}\right) = 2 \times 3 = 6$

They can be enumerated as:
 * $1, 2, 4, 5, 7, 8$