21722721 has 4 divisors (see below), whose sum is σ = 28963632. Its totient is φ = 14481812.

The previous prime is 21722717. The next prime is 21722747. The reversal of 21722721 is 12722712.

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 21722721 - 2² = 21722717 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21722691 and 21722700.

It is not an unprimeable number, because it can be changed into a prime (21722791) 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, 3620451 + ... + 3620456.

It is an arithmetic number, because the mean of its divisors is an integer number (7240908).

Almost surely, 2^21722721 is an apocalyptic number.

It is an amenable number.

21722721 is a deficient number, since it is larger than the sum of its proper divisors (7240911).

21722721 is an equidigital number, since it uses as much as digits as its factorization.

21722721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7240910.

The product of its digits is 784, while the sum is 24.

The square root of 21722721 is about 4660.7639931668. The cubic root of 21722721 is about 279.0217606131.

Subtracting from 21722721 its reverse (12722712), we obtain a palindrome (9000009).

The spelling of 21722721 in words is "twenty-one million, seven hundred twenty-two thousand, seven hundred twenty-one".