def _prime_factors(n):
  """ Returns the prime factors of n. Unmemoized version. """
  assert isinstance(n, int)

  result = []
  p=2
  while p**2 <= n:
    if n % p == 0:
      result.append(p)
      n /= p
    else:
      p += 1
  result.append(int(n))
  return result