for i in range(2, 20):

    for x in range(2, i):

        if i % x == 0:

            break

    else:

        print(i, "is a prime number")

​

def is_prime(n: int) -> bool:

    """Primality test using 6k+-1 optimization."""

    if n <= 3:

        return n > 1

    if n % 2 == 0 or n % 3 == 0:

        return False

    i = 5

    while i ** 2 <= n:

        if n % i == 0 or n % (i + 2) == 0:

            return False

        i += 6

    return True

​

def is_prime(n: int) -> bool:

    """Primality test using 6k+-1 optimization."""

    import math

    if n <= 3:

        return n > 1

    if n % 2 == 0 or n % 3 == 0:

        return False

    i = 5

    while i <= math.sqrt(n):

        if n % i == 0 or n % (i + 2) == 0:

            return False

        i += 6

    return True

num = 17

is_prime = f"number {num} is a prime number"

if num < 2:

    is_prime = False

else:

    for i in range(2, num):

        if num % i == 0:

            is_prime = f"number {num} is NOT a prime number"

            break

print(is_prime)