21921 has 4 divisors (see below), whose sum is σ = 29232. Its totient is φ = 14612.

The previous prime is 21911. The next prime is 21929. The reversal of 21921 is 12912.

It is a happy number.

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 not a de Polignac number, because 21921 - 2¹⁰ = 20897 is a prime.

It is a D-number.

It is a self number, because there is not a number n which added to its sum of digits gives 21921.

It is not an unprimeable number, because it can be changed into a prime (21929) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3651 + ... + 3656.

It is an arithmetic number, because the mean of its divisors is an integer number (7308).

2²¹⁹²¹ is an apocalyptic number.

It is an amenable number.

21921 is a deficient number, since it is larger than the sum of its proper divisors (7311).

21921 is an equidigital number, since it uses as much as digits as its factorization.

21921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7310.

The product of its digits is 36, while the sum is 15.

The square root of 21921 is about 148.0574212932. The cubic root of 21921 is about 27.9868135187.

Subtracting from 21921 its reverse (12912), we obtain a palindrome (9009).

The spelling of 21921 in words is "twenty-one thousand, nine hundred twenty-one".