21759721 has 2 divisors, whose sum is σ = 21759722. Its totient is φ = 21759720.

The previous prime is 21759713. The next prime is 21759757. The reversal of 21759721 is 12795712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 19263321 + 2496400 = 4389^2 + 1580^2 .

It is a cyclic number.

It is not a de Polignac number, because 21759721 - 2³ = 21759713 is a prime.

It is not a weakly prime, because it can be changed into another prime (21759761) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10879860 + 10879861.

It is an arithmetic number, because the mean of its divisors is an integer number (10879861).

Almost surely, 2^21759721 is an apocalyptic number.

It is an amenable number.

21759721 is a deficient number, since it is larger than the sum of its proper divisors (1).

21759721 is an equidigital number, since it uses as much as digits as its factorization.

21759721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 8820, while the sum is 34.

The square root of 21759721 is about 4664.7316107146. The cubic root of 21759721 is about 279.1800886792.

The spelling of 21759721 in words is "twenty-one million, seven hundred fifty-nine thousand, seven hundred twenty-one".