21900 has 36 divisors (see below), whose sum is σ = 64232. Its totient is φ = 5760.

The previous prime is 21893. The next prime is 21911. The reversal of 21900 is 912.

It is a happy number.

21900 is nontrivially palindromic in base 9.

It is a super-2 number, since 2×21900² = 959220000, which contains 22 as substring.

It is a Harshad number since it is a multiple of its sum of digits (12).

It is a plaindrome in base 16.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 264 + ... + 336.

2²¹⁹⁰⁰ is an apocalyptic number.

21900 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21900, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32116).

21900 is an abundant number, since it is smaller than the sum of its proper divisors (42332).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21900 is a wasteful number, since it uses less digits than its factorization.

21900 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 90 (or 83 counting only the distinct ones).

The product of its (nonzero) digits is 18, while the sum is 12.

The square root of 21900 is about 147.9864858695. The cubic root of 21900 is about 27.9778736763.

Adding to 21900 its sum of digits (12), we get a palindrome (21912).

The spelling of 21900 in words is "twenty-one thousand, nine hundred".