### 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 aeuler(n)=1 mod n