21529681 has 2 divisors, whose sum is σ = 21529682. Its totient is φ = 21529680.

The previous prime is 21529649. The next prime is 21529691. The reversal of 21529681 is 18692512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 21529600 + 81 = 4640^2 + 9^2 .

It is a cyclic number.

It is not a de Polignac number, because 21529681 - 2⁵ = 21529649 is a prime.

It is a super-2 number, since 2×21529681² = 927054327923522, which contains 22 as substring.

It is not a weakly prime, because it can be changed into another prime (21529621) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10764840 + 10764841.

It is an arithmetic number, because the mean of its divisors is an integer number (10764841).

Almost surely, 2^21529681 is an apocalyptic number.

It is an amenable number.

21529681 is a deficient number, since it is larger than the sum of its proper divisors (1).

21529681 is an equidigital number, since it uses as much as digits as its factorization.

21529681 is an evil number, because the sum of its binary digits is even.

The product of its digits is 8640, while the sum is 34.

The square root of 21529681 is about 4640.0087284401. The cubic root of 21529681 is about 278.1927867596.

It can be divided in two parts, 215296 and 81, that multiplied together give a square (17438976 = 4176²).

The spelling of 21529681 in words is "twenty-one million, five hundred twenty-nine thousand, six hundred eighty-one".