21905681 has 4 divisors (see below), whose sum is σ = 25035072. Its totient is φ = 18776292.

The previous prime is 21905647. The next prime is 21905687. The reversal of 21905681 is 18650912.

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 21905681 - 2⁶ = 21905617 is a prime.

It is a super-2 number, since 2×21905681² = 959717720147522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21905687) 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, 1564685 + ... + 1564698.

It is an arithmetic number, because the mean of its divisors is an integer number (6258768).

Almost surely, 2^21905681 is an apocalyptic number.

It is an amenable number.

21905681 is a deficient number, since it is larger than the sum of its proper divisors (3129391).

21905681 is an equidigital number, since it uses as much as digits as its factorization.

21905681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3129390.

The product of its (nonzero) digits is 4320, while the sum is 32.

The square root of 21905681 is about 4680.3505210614. The cubic root of 21905681 is about 279.8029268021.

The spelling of 21905681 in words is "twenty-one million, nine hundred five thousand, six hundred eighty-one".