input: Write a python function to check if a number is prime.
  assert is_prime(13) == True
  assert is_prime(4) == False
  assert is_prime(17) == True
  assert is_prime(1) == False
output: def is_prime(n):
      if n <= 1:
          return False
      if n <= 3:
          return True
      if n % 2 == 0 or n % 3 == 0:
          return False
      i = 5
      while i * i <= n:
          if n % i == 0 or n % (i + 2) == 0:
              return False
          i += 6
      return True