21840821 has 4 divisors (see below), whose sum is σ = 21944544. Its totient is φ = 21737100.

The previous prime is 21840817. The next prime is 21840829. The reversal of 21840821 is 12804812.

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 21840821 - 2² = 21840817 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21840829) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 51545 + ... + 51966.

It is an arithmetic number, because the mean of its divisors is an integer number (5486136).

Almost surely, 2^21840821 is an apocalyptic number.

It is an amenable number.

21840821 is a deficient number, since it is larger than the sum of its proper divisors (103723).

21840821 is a wasteful number, since it uses less digits than its factorization.

21840821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 103722.

The product of its (nonzero) digits is 1024, while the sum is 26.

The square root of 21840821 is about 4673.4164162848. The cubic root of 21840821 is about 279.5264998988.

The spelling of 21840821 in words is "twenty-one million, eight hundred forty thousand, eight hundred twenty-one".