21864721 has 4 divisors (see below), whose sum is σ = 21909744. Its totient is φ = 21819700.

The previous prime is 21864659. The next prime is 21864757. The reversal of 21864721 is 12746812.

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 21864721 - 2⁹ = 21864209 is a prime.

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21864761) 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, 21775 + ... + 22756.

It is an arithmetic number, because the mean of its divisors is an integer number (5477436).

Almost surely, 2^21864721 is an apocalyptic number.

It is an amenable number.

21864721 is a deficient number, since it is larger than the sum of its proper divisors (45023).

21864721 is an equidigital number, since it uses as much as digits as its factorization.

21864721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 45022.

The product of its digits is 5376, while the sum is 31.

The square root of 21864721 is about 4675.9727330257. The cubic root of 21864721 is about 279.6284229290.

The spelling of 21864721 in words is "twenty-one million, eight hundred sixty-four thousand, seven hundred twenty-one".