21621 has 4 divisors (see below), whose sum is σ = 28832. Its totient is φ = 14412.

The previous prime is 21617. The next prime is 21647. The reversal of 21621 is 12612.

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 not a de Polignac number, because 21621 - 2² = 21617 is a prime.

It is a D-number.

It is a Duffinian number.

21621 is a lucky number.

It is a junction number, because it is equal to n+sod(n) for n = 21597 and 21606.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21601) 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, 3601 + ... + 3606.

It is an arithmetic number, because the mean of its divisors is an integer number (7208).

2²¹⁶²¹ is an apocalyptic number.

It is an amenable number.

21621 is a deficient number, since it is larger than the sum of its proper divisors (7211).

21621 is an equidigital number, since it uses as much as digits as its factorization.

21621 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7210.

The product of its digits is 24, while the sum is 12.

The square root of 21621 is about 147.0408106615. The cubic root of 21621 is about 27.8585553900.

Subtracting from 21621 its sum of digits (12), we obtain a square (21609 = 147²).

Subtracting from 21621 its reverse (12612), we obtain a palindrome (9009).

It can be divided in two parts, 21 and 621, that multiplied together give a triangular number (13041 = T₁₆₁).

The spelling of 21621 in words is "twenty-one thousand, six hundred twenty-one".