21159821 has 4 divisors (see below), whose sum is σ = 21889500. Its totient is φ = 20430144.

The previous prime is 21159811. The next prime is 21159857. The reversal of 21159821 is 12895112.

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 12075625 + 9084196 = 3475^2 + 3014^2 .

It is a cyclic number.

It is not a de Polignac number, because 21159821 - 2⁶ = 21159757 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (29), and also a Moran number because the ratio is a prime number: 729649 = 21159821 / (2 + 1 + 1 + 5 + 9 + 8 + 2 + 1).

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21159811) 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, 364796 + ... + 364853.

It is an arithmetic number, because the mean of its divisors is an integer number (5472375).

Almost surely, 2^21159821 is an apocalyptic number.

It is an amenable number.

21159821 is a deficient number, since it is larger than the sum of its proper divisors (729679).

21159821 is an equidigital number, since it uses as much as digits as its factorization.

21159821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 729678.

The product of its digits is 1440, while the sum is 29.

The square root of 21159821 is about 4599.9805434371. The cubic root of 21159821 is about 276.5905448552.

The spelling of 21159821 in words is "twenty-one million, one hundred fifty-nine thousand, eight hundred twenty-one".