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.

Find out more:

What is a prime number?

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:

15 is a multiple of 1
15 is a multiple of 3
15 is a multiple of 5

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).

Parity of 15

15 is an odd number, because it is not evenly divisible by 2.

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What is an even number?

Is 15 a perfect square number?

A number is a perfect square (or a square number) if its square root is an integer; that is to say, it is the product of an integer with itself. Here, the square root of 15 is about 3.873.

Thus, the square root of 15 is not an integer, and therefore 15 is not a square number.

What is the square number of 15?

The square of a number (here 15) is the result of the product of this number (15) by itself (i.e., 15 × 15); the square of 15 is sometimes called "raising 15 to the power 2", or "15 squared".

The square of 15 is 225 because 15 × 15 = 15² = 225.

As a consequence, 15 is the square root of 225.

Number of digits of 15

15 is a number with 2 digits.

What are the multiples of 15?

The multiples of 15 are all integers evenly divisible by 15, that is all numbers such that the remainder of the division by 15 is zero. There are infinitely many multiples of 15. The smallest multiples of 15 are:

0: indeed, 0 is divisible by any natural number, and it is thus a multiple of 15 too, since 0 × 15 = 0
15: indeed, 15 is a multiple of itself, since 15 is evenly divisible by 15 (we have 15 / 15 = 1, so the remainder of this division is indeed zero)
30: indeed, 30 = 15 × 2
45: indeed, 45 = 15 × 3
60: indeed, 60 = 15 × 4
75: indeed, 75 = 15 × 5
etc.

How to determine whether an integer is a prime number?

To determine the primality of a number, several algorithms can be used. The most naive technique is to test all divisors strictly smaller to the number of which we want to determine the primality (here 15). First, we can eliminate all even numbers greater than 2 (and hence 4, 6, 8…). Then, we can stop this check when we reach the square root of the number of which we want to determine the primality (here the square root is about 3.873). Historically, the sieve of Eratosthenes (dating from the Greek mathematics) implements this technique in a relatively efficient manner.

More modern techniques include the sieve of Atkin, probabilistic algorithms, and the cyclotomic AKS test.

Numbers near 15

Preceding numbers: …13, 14
Following numbers: 16, 17…

Nearest numbers from 15

Preceding prime number: 13
Following prime number: 17