21681631 has 4 divisors (see below), whose sum is σ = 21768264. Its totient is φ = 21595000.

The previous prime is 21681619. The next prime is 21681643. The reversal of 21681631 is 13618612.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21681619) and next prime (21681643).

It is a cyclic number.

It is not a de Polignac number, because 21681631 - 2⁵ = 21681599 is a prime.

It is a super-2 number, since 2×21681631² = 940186245640322, which contains 22 as substring.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21681631.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21681031) 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, 42940 + ... + 43441.

It is an arithmetic number, because the mean of its divisors is an integer number (5442066).

Almost surely, 2^21681631 is an apocalyptic number.

21681631 is a deficient number, since it is larger than the sum of its proper divisors (86633).

21681631 is an equidigital number, since it uses as much as digits as its factorization.

21681631 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 86632.

The product of its digits is 1728, while the sum is 28.

The square root of 21681631 is about 4656.3538310571. The cubic root of 21681631 is about 278.8457200410.

The spelling of 21681631 in words is "twenty-one million, six hundred eighty-one thousand, six hundred thirty-one".