21705681 has 8 divisors (see below), whose sum is σ = 29556864. Its totient is φ = 14162480.

The previous prime is 21705679. The next prime is 21705689. The reversal of 21705681 is 18650712.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 21705681 - 2¹ = 21705679 is a prime.

It is a super-2 number, since 2×21705681² = 942273175347522, which contains 22 as substring.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21705689) 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 in 7 ways as a sum of consecutive naturals, for example, 76830 + ... + 77111.

It is an arithmetic number, because the mean of its divisors is an integer number (3694608).

Almost surely, 2^21705681 is an apocalyptic number.

It is an amenable number.

21705681 is a deficient number, since it is larger than the sum of its proper divisors (7851183).

21705681 is a wasteful number, since it uses less digits than its factorization.

21705681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 153991.

The product of its (nonzero) digits is 3360, while the sum is 30.

The square root of 21705681 is about 4658.9356080547. The cubic root of 21705681 is about 278.9487836512.

The spelling of 21705681 in words is "twenty-one million, seven hundred five thousand, six hundred eighty-one".