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 :
Output :
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: