21931821 has 12 divisors (see below), whose sum is σ = 31724160. Its totient is φ = 14600520.

The previous prime is 21931817. The next prime is 21931823. The reversal of 21931821 is 12813912.

It is not a de Polignac number, because 21931821 - 2² = 21931817 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21931823) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 7690 + ... + 10148.

It is an arithmetic number, because the mean of its divisors is an integer number (2643680).

Almost surely, 2^21931821 is an apocalyptic number.

It is an amenable number.

21931821 is a deficient number, since it is larger than the sum of its proper divisors (9792339).

21931821 is a wasteful number, since it uses less digits than its factorization.

21931821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3456 (or 3453 counting only the distinct ones).

The product of its digits is 864, while the sum is 27.

The square root of 21931821 is about 4683.1422143685. The cubic root of 21931821 is about 279.9141786300.

The spelling of 21931821 in words is "twenty-one million, nine hundred thirty-one thousand, eight hundred twenty-one".