• 15 can be written using four 4's:

See also 113.

• 15² = 225 is the smallest square that contains exactly two digits '2'.

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15 has 4 divisors (see below), whose sum is σ = 24. Its totient is φ = 8.

The previous prime is 13. The next prime is 17. The reversal of 15 is 51.

Added to its reverse (51) it gives a triangular number (66 = T₁₁).

It is a double factorial (15 = 5 !! = 1 ⋅ 3 ⋅ 5 ).

15 is nontrivially palindromic in base 2, base 4 and base 14.

15 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

It is a Cunningham number, because it is equal to 2⁴-1.

15 is an esthetic number in base 6, base 7, base 13 and base 15, because in such bases its adjacent digits differ by 1.

15 is a nontrivial binomial coefficient, being equal to C(6, 2).

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 51 = 3 ⋅17.

It is an interprime number because it is at equal distance from previous prime (13) and next prime (17).

It is a 3-Lehmer number, since φ(15) divides (15-1)³.

It is a cyclic number.

It is the 4-th Bell number.

15 is an astonishing number since 15 = 1 + ... + 5.

It is not a de Polignac number, because 15 - 2¹ = 13 is a prime.

It is a nude number because it is divisible by every one of its digits and also a Zuckerman number because it is divisible by the product of its digits.

15 is an idoneal number.

It is a tetranacci number.

It is a D-number.

It is a cake number, because a cake can be divided into 15 parts by 4 planar cuts.

It is one of the 548 Lynch-Bell numbers.

15 is strictly pandigital in base 3.

It is a partition number, being equal to the number of ways a set of 7 identical objects can be partitioned into subset.

15 is a lucky number.

15 is a nontrivial repdigit in base 2, base 4 and base 14.

It is a plaindrome in base 2, base 4, base 6, base 8, base 9, base 10, base 11, base 12, base 13 and base 14.

It is a nialpdrome in base 2, base 4, base 5, base 7, base 14 and base 15.

It is a zygodrome in base 2, base 4 and base 14.

It is a congruent number.

It is a panconsummate number.

It is a nontrivial repunit in base 2.

A polygon with 15 sides can be constructed with ruler and compass.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 5.

It is an arithmetic number, because the mean of its divisors is an integer number (6).

It is the magic constant of a 3 × 3 magic square.

15 is the 5-th triangular number and also the 3-rd hexagonal number.

15 is a deficient number, since it is larger than the sum of its proper divisors (9).

15 is an equidigital number, since it uses as much as digits as its factorization.

With its successor (16) it forms a Ruth-Aaron pair, since the sum of their prime factors is the same (8).

15 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 8.

The product of its digits is 5, while the sum is 6.

The square root of 15 is about 3.8729833462. The cubic root of 15 is about 2.4662120743.

Adding to 15 its reverse (51), we get a palindrome (66).

Subtracting 15 from its reverse (51), we obtain a triangular number (36 = T₈).

The spelling of 15 in words is "fifteen", and thus it is an aban number, an oban number, and an uban number.