Euler Phi Function/Examples/Phi is 6

Example of Use of Euler $\phi$ Function
There are $4$ numbers for which the value of the Euler $\phi$ function is $6$:
 * $7, 9, 14, 18$

Proof
From Euler Phi Function of $9$, we already have that $\phi \left({9}\right) = 6$.

$7$
From Euler Phi Function of Prime:
 * $\phi \left({7}\right) = 7 - 1 = 6$

$14$
From Euler Phi Function of Integer:

$18$
From Euler Phi Function of Integer:

Hence the result.