21928721 has 2 divisors, whose sum is σ = 21928722. Its totient is φ = 21928720.

The previous prime is 21928663. The next prime is 21928723. The reversal of 21928721 is 12782912.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17472400 + 4456321 = 4180^2 + 2111^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21928721 is a prime.

Together with 21928723, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (21928723) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10964360 + 10964361.

It is an arithmetic number, because the mean of its divisors is an integer number (10964361).

Almost surely, 2^21928721 is an apocalyptic number.

It is an amenable number.

21928721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21928721 is an equidigital number, since it uses as much as digits as its factorization.

21928721 is an evil number, because the sum of its binary digits is even.

The product of its digits is 4032, while the sum is 32.

The square root of 21928721 is about 4682.8112283115. The cubic root of 21928721 is about 279.9009896531.

Adding to 21928721 its sum of digits (32), we get a triangular number (21928753 = T₆₆₂₂).

The spelling of 21928721 in words is "twenty-one million, nine hundred twenty-eight thousand, seven hundred twenty-one".