21631 has 4 divisors (see below), whose sum is σ = 21952. Its totient is φ = 21312.

The previous prime is 21617. The next prime is 21647. The reversal of 21631 is 13612.

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

It is a cyclic number.

It is not a de Polignac number, because 21631 - 2⁵ = 21599 is a prime.

It is a super-2 number, since 2×21631² = 935800322, which contains 22 as substring.

It is a Duffinian number.

It is a plaindrome in base 13.

It is a nialpdrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21601) 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, 15 + ... + 208.

It is an arithmetic number, because the mean of its divisors is an integer number (5488).

2²¹⁶³¹ is an apocalyptic number.

21631 is a deficient number, since it is larger than the sum of its proper divisors (321).

21631 is an equidigital number, since it uses as much as digits as its factorization.

21631 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 320.

The product of its digits is 36, while the sum is 13.

The square root of 21631 is about 147.0748108957. The cubic root of 21631 is about 27.8628497121.

The spelling of 21631 in words is "twenty-one thousand, six hundred thirty-one".