# Is 15 a prime number?

It is possible to find out using mathematical methods whether a given integer is a prime number or not.

For 15, the answer is: No, 15 is not a prime number.

The list of all positive divisors (i.e., the list of all integers that divide 15) is as follows: 1, 3, 5, 15.

For 15 to be a prime number, it would have been required that 15 has only two divisors, i.e., itself and 1.

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Actually, one can immediately see that 15 cannot be prime, because 5 is one of its divisors: indeed, a number ending with 0 or 5 has necessarily 5 among its divisors.
The last digit of 15 is 5, so it is divisible by 5 and is therefore *not* prime.

As a consequence:

For 15 to be a prime number, it would have been required that 15 has only two divisors, i.e., itself and 1.

However, 15 is a **semiprime** (also called biprime or 2-almost-prime), because it is the product of a two non-necessarily distinct prime numbers. Indeed, 15 = 3 x 5, where 3 and 5 are both prime numbers.

### Is 15 a deficient number?

Yes, 15 is a deficient number, that is to say 15 is a natural number that is strictly larger than the sum of its proper divisors, i.e., the divisors of 15 without 15 itself (that is 1 + 3 + 5 = 9).