21821 has 2 divisors, whose sum is σ = 21822. Its totient is φ = 21820.

The previous prime is 21817. The next prime is 21839. The reversal of 21821 is 12812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19321 + 2500 = 139^2 + 50^2 .

It is a cyclic number.

It is not a de Polignac number, because 21821 - 2² = 21817 is a prime.

It is a Chen prime.

It is a junction number, because it is equal to n+sod(n) for n = 21796 and 21805.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21841) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10910 + 10911.

It is an arithmetic number, because the mean of its divisors is an integer number (10911).

2²¹⁸²¹ is an apocalyptic number.

It is an amenable number.

21821 is a deficient number, since it is larger than the sum of its proper divisors (1).

21821 is an equidigital number, since it uses as much as digits as its factorization.

21821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 32, while the sum is 14.

The square root of 21821 is about 147.7193284577. The cubic root of 21821 is about 27.9441915599.

Subtracting from 21821 its reverse (12812), we obtain a palindrome (9009).

The spelling of 21821 in words is "twenty-one thousand, eight hundred twenty-one".