21989 has 4 divisors (see below), whose sum is σ = 24000. Its totient is φ = 19980.

The previous prime is 21977. The next prime is 21991. The reversal of 21989 is 98912.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21989 is a prime.

It is a super-2 number, since 2×21989² = 967032242, which contains 22 as substring.

It is a Duffinian number.

It is a plaindrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21929) 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, 989 + ... + 1010.

It is an arithmetic number, because the mean of its divisors is an integer number (6000).

2²¹⁹⁸⁹ is an apocalyptic number.

It is an amenable number.

21989 is a deficient number, since it is larger than the sum of its proper divisors (2011).

21989 is a wasteful number, since it uses less digits than its factorization.

21989 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2010.

The product of its digits is 1296, while the sum is 29.

The square root of 21989 is about 148.2868841132. The cubic root of 21989 is about 28.0157224624.

It can be divided in two parts, 219 and 89, that multiplied together give a palindrome (19491).

The spelling of 21989 in words is "twenty-one thousand, nine hundred eighty-nine".