The Euler indicatrix: euler phi

### 5.5.24 The Euler indicatrix: euler phi

The Euler phi function (also called the Euler totient function) finds the number of positive integers less than a given integer and relatively prime. The euler command computes the Euler phi function.

- euler takes one argument:
  n, a non-negative integer.
- euler(n) returns the number of integers larger than 1, less than n and relatively prime to n.

#### Example.

Input:

euler(21)

Output:

12

In other words the set of integers less than 21 and coprime with 21, {1,2,4,5,8,10,11,13,16,17,19,20}, has 12 elements.

The little Fermat theorem states:

If p is a prime number, then for any integer a, ap−1=1 modp.

Euler introduced his phi function to generalize the little Fermat theorem:

If a and n are relatively prime, then aeuler(n)=1 mod n.

#### Example.

Input:

powmod(5,12,21)

(see section Section 5.34.10)
Output:

1