www.reference.com/article/1-prime-number-d0c22610f491fbc4

The number 1 is not a prime number. A prime number by definition is a positive integer greater than 1 that is evenly divisible by only itself and 1.

www.reference.com/article/2-prime-number-c4622f275383f243

Yes, 2 is a prime number. In fact, 2 is the only even number that is prime. A prime number is a number that has only two factors, the number 1 and itself.

www.reference.com/article/15-prime-number-9c4b015fd8478e0f

Fifteen is not a prime number. A prime number is a positive number that has only two possible divisors, 1 and itself. Fifteen can be divided by 3 and 5 as well as 1 and itself.

www.reference.com/article/two-prime-number-e424a94a55b2cf3a

Two is a prime number. Two is the smallest prime number and the only even prime number, since all other even numbers divide by two.

www.reference.com/article/one-prime-number-4c916ba7da91e2fa

One is not a prime number because the definition of a prime number is any number that is greater than one that is only divisible by one and itself. Since one is of course not greater than one, it does not fit the definition of a prime number.

www.reference.com/article/21-prime-number-12e5677deb89b62

The number 21 is not a prime number. Prime numbers are numbers greater than one that are evenly divisible by only 1 and themselves. As the number 21 has multiple factors, including 1, 21, 3 and 7, it does not meet the definition of a prime number.

www.reference.com/article/19-prime-number-fda5f29d99e14071

The number 19 is the eighth prime number. A prime number is a positive, whole number that is only divisible by one and itself. Nineteen is also the seventh Mersenne prime exponent, which is a prime number that is one less than a power of two.