21687 has 4 divisors (see below), whose sum is σ = 28920. Its totient is φ = 14456.

The previous prime is 21683. The next prime is 21701. The reversal of 21687 is 78612.

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 21687 - 2² = 21683 is a prime.

It is an Ulam number.

It is a D-number.

21687 is a lucky number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21687.

It is not an unprimeable number, because it can be changed into a prime (21683) 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, 3612 + ... + 3617.

It is an arithmetic number, because the mean of its divisors is an integer number (7230).

2²¹⁶⁸⁷ is an apocalyptic number.

21687 is a deficient number, since it is larger than the sum of its proper divisors (7233).

21687 is an equidigital number, since it uses as much as digits as its factorization.

21687 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7232.

The product of its digits is 672, while the sum is 24.

The square root of 21687 is about 147.2650671409. The cubic root of 21687 is about 27.8868734901.

It can be divided in two parts, 216 and 87, that added together give a palindrome (303).

The spelling of 21687 in words is "twenty-one thousand, six hundred eighty-seven".