21868721 has 4 divisors (see below), whose sum is σ = 24992832. Its totient is φ = 18744612.

The previous prime is 21868709. The next prime is 21868723. The reversal of 21868721 is 12786812.

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 21868721 - 2⁶ = 21868657 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21868723) 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, 1562045 + ... + 1562058.

It is an arithmetic number, because the mean of its divisors is an integer number (6248208).

Almost surely, 2^21868721 is an apocalyptic number.

It is an amenable number.

21868721 is a deficient number, since it is larger than the sum of its proper divisors (3124111).

21868721 is an equidigital number, since it uses as much as digits as its factorization.

21868721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3124110.

The product of its digits is 10752, while the sum is 35.

The square root of 21868721 is about 4676.4004319562. The cubic root of 21868721 is about 279.6454739201.

The spelling of 21868721 in words is "twenty-one million, eight hundred sixty-eight thousand, seven hundred twenty-one".