21815 has 4 divisors (see below), whose sum is σ = 26184. Its totient is φ = 17448.

The previous prime is 21803. The next prime is 21817. The reversal of 21815 is 51812.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21815 - 2⁴ = 21799 is a prime.

It is a Duffinian number.

It is a plaindrome in base 6.

It is a junction number, because it is equal to n+sod(n) for n = 21793 and 21802.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21817) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2177 + ... + 2186.

It is an arithmetic number, because the mean of its divisors is an integer number (6546).

2²¹⁸¹⁵ is an apocalyptic number.

21815 is a deficient number, since it is larger than the sum of its proper divisors (4369).

21815 is an equidigital number, since it uses as much as digits as its factorization.

21815 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4368.

The product of its digits is 80, while the sum is 17.

The square root of 21815 is about 147.6990182770. The cubic root of 21815 is about 27.9416301051.

The spelling of 21815 in words is "twenty-one thousand, eight hundred fifteen".