21831 has 8 divisors (see below), whose sum is σ = 30720. Its totient is φ = 13752.

The previous prime is 21821. The next prime is 21839. The reversal of 21831 is 13812.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 21831 - 2⁵ = 21799 is a prime.

It is a super-2 number, since 2×21831² = 953185122, which contains 22 as substring.

It is a self number, because there is not a number n which added to its sum of digits gives 21831.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21839) 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, 135 + ... + 248.

It is an arithmetic number, because the mean of its divisors is an integer number (3840).

2²¹⁸³¹ is an apocalyptic number.

21831 is a deficient number, since it is larger than the sum of its proper divisors (8889).

21831 is a wasteful number, since it uses less digits than its factorization.

21831 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 405.

The product of its digits is 48, while the sum is 15.

The square root of 21831 is about 147.7531725548. The cubic root of 21831 is about 27.9484596081.

The spelling of 21831 in words is "twenty-one thousand, eight hundred thirty-one".