21687681 has 8 divisors (see below), whose sum is σ = 28939680. Its totient is φ = 14447072.

The previous prime is 21687641. The next prime is 21687691. The reversal of 21687681 is 18678612.

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 21687681 - 2⁷ = 21687553 is a prime.

It is a super-2 number, since 2×21687681² = 940711014315522, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21687641) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 3850 + ... + 7628.

It is an arithmetic number, because the mean of its divisors is an integer number (3617460).

Almost surely, 2^21687681 is an apocalyptic number.

It is an amenable number.

21687681 is a deficient number, since it is larger than the sum of its proper divisors (7251999).

21687681 is a wasteful number, since it uses less digits than its factorization.

21687681 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5695.

The product of its digits is 32256, while the sum is 39.

The square root of 21687681 is about 4657.0034356869. The cubic root of 21687681 is about 278.8716538175.

The spelling of 21687681 in words is "twenty-one million, six hundred eighty-seven thousand, six hundred eighty-one".