21523681 has 2 divisors, whose sum is σ = 21523682. Its totient is φ = 21523680.

The previous prime is 21523679. The next prime is 21523727. The reversal of 21523681 is 18632512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 21307456 + 216225 = 4616^2 + 465^2 .

It is a cyclic number.

It is not a de Polignac number, because 21523681 - 2¹ = 21523679 is a prime.

It is a super-2 number, since 2×21523681² = 926537687579522, which contains 22 as substring.

Together with 21523679, it forms a pair of twin primes.

It is not a weakly prime, because it can be changed into another prime (21523631) 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, 10761840 + 10761841.

It is an arithmetic number, because the mean of its divisors is an integer number (10761841).

Almost surely, 2^21523681 is an apocalyptic number.

It is an amenable number.

21523681 is a deficient number, since it is larger than the sum of its proper divisors (1).

21523681 is an equidigital number, since it uses as much as digits as its factorization.

21523681 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 2880, while the sum is 28.

The square root of 21523681 is about 4639.3621328799. The cubic root of 21523681 is about 278.1669416359.

The spelling of 21523681 in words is "twenty-one million, five hundred twenty-three thousand, six hundred eighty-one".