21681681 has 16 divisors (see below), whose sum is σ = 34982784. Its totient is φ = 11660544.

The previous prime is 21681659. The next prime is 21681683. The reversal of 21681681 is 18618612.

It is not a de Polignac number, because 21681681 - 2⁷ = 21681553 is a prime.

It is a super-2 number, since 2×21681681² = 940190581971522, which contains 22 as substring.

It is not an unprimeable number, because it can be changed into a prime (21681683) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 30010 + ... + 30723.

It is an arithmetic number, because the mean of its divisors is an integer number (2186424).

Almost surely, 2^21681681 is an apocalyptic number.

21681681 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

It is an amenable number.

21681681 is a deficient number, since it is larger than the sum of its proper divisors (13301103).

21681681 is a wasteful number, since it uses less digits than its factorization.

21681681 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 60760.

The product of its digits is 4608, while the sum is 33.

The square root of 21681681 is about 4656.3592000618. The cubic root of 21681681 is about 278.8459343895.

The spelling of 21681681 in words is "twenty-one million, six hundred eighty-one thousand, six hundred eighty-one".