21605681 has 4 divisors (see below), whose sum is σ = 21628992. Its totient is φ = 21582372.

The previous prime is 21605677. The next prime is 21605693. The reversal of 21605681 is 18650612.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 21605681 - 2² = 21605677 is a prime.

It is a super-2 number, since 2×21605681² = 933610902947522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21605611) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 10205 + ... + 12138.

It is an arithmetic number, because the mean of its divisors is an integer number (5407248).

Almost surely, 2^21605681 is an apocalyptic number.

It is an amenable number.

21605681 is a deficient number, since it is larger than the sum of its proper divisors (23311).

21605681 is an equidigital number, since it uses as much as digits as its factorization.

21605681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 23310.

The product of its (nonzero) digits is 2880, while the sum is 29.

The square root of 21605681 is about 4648.1911535564. The cubic root of 21605681 is about 278.5197434934.

The spelling of 21605681 in words is "twenty-one million, six hundred five thousand, six hundred eighty-one".