21840 has 80 divisors (see below), whose sum is σ = 83328. Its totient is φ = 4608.

The previous prime is 21839. The next prime is 21841. The reversal of 21840 is 4812.

It is an interprime number because it is at equal distance from previous prime (21839) and next prime (21841).

It is a tau number, because it is divible by the number of its divisors (80).

It is a Harshad number since it is a multiple of its sum of digits (15).

It is a nialpdrome in base 4 and base 16.

It is a zygodrome in base 4.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21841) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 1674 + ... + 1686.

2²¹⁸⁴⁰ is an apocalyptic number.

21840 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 21840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (41664).

21840 is an abundant number, since it is smaller than the sum of its proper divisors (61488).

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

21840 is a wasteful number, since it uses less digits than its factorization.

21840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 36 (or 30 counting only the distinct ones).

The product of its (nonzero) digits is 64, while the sum is 15.

The square root of 21840 is about 147.7836256153. The cubic root of 21840 is about 27.9522997373.

Adding to 21840 its product of nonzero digits (64), we get a square (21904 = 148²).

It can be divided in two parts, 21 and 840, that added together give a triangular number (861 = T₄₁).

The spelling of 21840 in words is "twenty-one thousand, eight hundred forty".