21686681 has 4 divisors (see below), whose sum is σ = 21696000. Its totient is φ = 21677364.

The previous prime is 21686671. The next prime is 21686741. The reversal of 21686681 is 18668612.

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, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21686681 - 2⁶ = 21686617 is a prime.

It is a super-2 number, since 2×21686681² = 940624265591522, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21686671) 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 3 ways as a sum of consecutive naturals, for example, 2120 + ... + 6918.

It is an arithmetic number, because the mean of its divisors is an integer number (5424000).

Almost surely, 2^21686681 is an apocalyptic number.

It is an amenable number.

21686681 is a deficient number, since it is larger than the sum of its proper divisors (9319).

21686681 is an equidigital number, since it uses as much as digits as its factorization.

21686681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9318.

The product of its digits is 27648, while the sum is 38.

The square root of 21686681 is about 4656.8960692719. The cubic root of 21686681 is about 278.8673675756.

The spelling of 21686681 in words is "twenty-one million, six hundred eighty-six thousand, six hundred eighty-one".