input: from typing import List
  
  def is_prime_number(number: int) -> str:
      """ Determine if a given number is prime or not. If the number is prime, return "{number} is a prime number.", otherwise return "{number} is not a prime number."
      >>> is_prime_number(17)
      '17 is a prime number.'
      >>> is_prime_number(15)
      '15 is not a prime number.'
      >>> is_prime_number(2)
      '2 is a prime number.'
      """
output:     if number <= 1:
          return f"{number} is not a prime number."
      for i in range(2, int(number**0.5) + 1):
          if number % i == 0:
              return f"{number} is not a prime number."
      return f"{number} is a prime number."