F f(n)
   R sum((1..n).filter(k -> gcd(@n, k) == 1).map(k -> 1))

F is_prime(n)
   R f(n) == n - 1

L(n) 1..25
   print(‘ f(#.) == #.’.format(n, f(n))‘’(I is_prime(n) {‘, is prime’} E ‘’))
V count = 0
L(n) 1..10'000
   count += is_prime(n)
   I n C (100, 1000, 10'000)
      print(‘Primes up to #.: #.’.format(n, count))