21279721 has 2 divisors, whose sum is σ = 21279722. Its totient is φ = 21279720.

The previous prime is 21279703. The next prime is 21279737. The reversal of 21279721 is 12797212.

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., 19263321 + 2016400 = 4389^2 + 1420^2 .

It is a cyclic number.

It is not a de Polignac number, because 21279721 - 2⁵ = 21279689 is a prime.

It is not a weakly prime, because it can be changed into another prime (21279701) 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 as a sum of consecutive naturals, namely, 10639860 + 10639861.

It is an arithmetic number, because the mean of its divisors is an integer number (10639861).

Almost surely, 2^21279721 is an apocalyptic number.

It is an amenable number.

21279721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21279721 is an equidigital number, since it uses as much as digits as its factorization.

21279721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3528, while the sum is 31.

The square root of 21279721 is about 4612.9947973090. The cubic root of 21279721 is about 277.1119853739. Note that the first 3 decimals are identical.

The spelling of 21279721 in words is "twenty-one million, two hundred seventy-nine thousand, seven hundred twenty-one".