21685 has 4 divisors (see below), whose sum is σ = 26028. Its totient is φ = 17344.

The previous prime is 21683. The next prime is 21701. The reversal of 21685 is 58612.

It is a happy number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 2916 + 18769 = 54^2 + 137^2 .

It is a cyclic number.

It is not a de Polignac number, because 21685 - 2¹ = 21683 is a prime.

It is a Smith number, since the sum of its digits (22) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax 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 3 ways as a sum of consecutive naturals, for example, 2164 + ... + 2173.

It is an arithmetic number, because the mean of its divisors is an integer number (6507).

2²¹⁶⁸⁵ is an apocalyptic number.

It is an amenable number.

21685 is a deficient number, since it is larger than the sum of its proper divisors (4343).

21685 is an equidigital number, since it uses as much as digits as its factorization.

21685 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4342.

The product of its digits is 480, while the sum is 22.

The square root of 21685 is about 147.2582765076. The cubic root of 21685 is about 27.8860162106.

The spelling of 21685 in words is "twenty-one thousand, six hundred eighty-five".