21987 has 12 divisors (see below), whose sum is σ = 36400. Its totient is φ = 12528.

The previous prime is 21977. The next prime is 21991. The reversal of 21987 is 78912.

It is not a de Polignac number, because 21987 - 2⁷ = 21859 is a prime.

It is an Ulam number.

21987 is a lucky number.

It is a plaindrome in base 11 and base 15.

It is a self number, because there is not a number n which added to its sum of digits gives 21987.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21987.

It is not an unprimeable number, because it can be changed into a prime (21937) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 112 + ... + 237.

2²¹⁹⁸⁷ is an apocalyptic number.

21987 is a deficient number, since it is larger than the sum of its proper divisors (14413).

21987 is a wasteful number, since it uses less digits than its factorization.

21987 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 362 (or 359 counting only the distinct ones).

The product of its digits is 1008, while the sum is 27.

The square root of 21987 is about 148.2801402751. The cubic root of 21987 is about 28.0148730507.

The spelling of 21987 in words is "twenty-one thousand, nine hundred eighty-seven".