def ec_phi(n, d):
if is_prime(n):
return n-1
if not n % d:
if is_prime(d):
q = n / d
return (d-1)*ec_phi(q, int(q**.5) + 1)
return ec_phi(n, d-1)
def is_strong(n):
d = digit_sum(n)
if is_prime(n/d):
return True
return False
