### 2.6.25 The Euler indicatrix : `euler phi`

`euler` (or `phi`) returns the Euler indicatrix
for a integer.

`euler(n)` (or `phi(n)`) is equal to the number of integers less
than `n` and prime with `n`.

Input :

`euler(21)`

Output :

`12`

In other words
E={2,4,5,7,8,10,11,13,15,16,17,19} is the set of integers less than 21
and coprime with 21. There are 12 members in this set, hence Cardinal(E)=12.

Euler has introduced this function to generalize the little Fermat theorem:

If *a* and *n* are mutually prime then *a*^{euler(n)}=1 mod *n*