21689 has 6 divisors (see below), whose sum is σ = 23226. Its totient is φ = 20240.

The previous prime is 21683. The next prime is 21701. The reversal of 21689 is 98612.

It can be written as a sum of positive squares in only one way, i.e., 13225 + 8464 = 115^2 + 92^2 .

It is not a de Polignac number, because 21689 - 2⁴ = 21673 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is a congruent number.

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 5 ways as a sum of consecutive naturals, for example, 509 + ... + 549.

It is an arithmetic number, because the mean of its divisors is an integer number (3871).

2²¹⁶⁸⁹ is an apocalyptic number.

It is an amenable number.

21689 is a deficient number, since it is larger than the sum of its proper divisors (1537).

21689 is an equidigital number, since it uses as much as digits as its factorization.

21689 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 87 (or 64 counting only the distinct ones).

The product of its digits is 864, while the sum is 26.

The square root of 21689 is about 147.2718574610. The cubic root of 21689 is about 27.8877307169.

The spelling of 21689 in words is "twenty-one thousand, six hundred eighty-nine".