21800 has 24 divisors (see below), whose sum is σ = 51150. Its totient is φ = 8640.

The previous prime is 21799. The next prime is 21803. The reversal of 21800 is 812.

21800 = T₄₄ + T₄₅ + ... + T₅₉.

It can be written as a sum of positive squares in 3 ways, for example, as 17956 + 3844 = 134^2 + 62^2 .

It is a plaindrome in base 13.

It is not an unprimeable number, because it can be changed into a prime (21803) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 146 + ... + 254.

2²¹⁸⁰⁰ is an apocalyptic number.

21800 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 21800, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25575).

21800 is an abundant number, since it is smaller than the sum of its proper divisors (29350).

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

21800 is a wasteful number, since it uses less digits than its factorization.

21800 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 125 (or 116 counting only the distinct ones).

The product of its (nonzero) digits is 16, while the sum is 11.

The square root of 21800 is about 147.6482306023. The cubic root of 21800 is about 27.9352244125.

It can be divided in two parts, 2 and 1800, that multiplied together give a square (3600 = 60²).

The spelling of 21800 in words is "twenty-one thousand, eight hundred".