21644 has 12 divisors (see below), whose sum is σ = 43344. Its totient is φ = 9264.

The previous prime is 21617. The next prime is 21647. The reversal of 21644 is 44612.

21644 is an admirable number.

It is not an unprimeable number, because it can be changed into a prime (21647) 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, 359 + ... + 414.

It is an arithmetic number, because the mean of its divisors is an integer number (3612).

2²¹⁶⁴⁴ is an apocalyptic number.

It is an amenable number.

21644 is a primitive abundant number, since it is smaller than the sum of its proper divisors, none of which is abundant.

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

It is a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (21672).

21644 is a wasteful number, since it uses less digits than its factorization.

21644 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 784 (or 782 counting only the distinct ones).

The product of its digits is 192, while the sum is 17.

The square root of 21644 is about 147.1189994528. The cubic root of 21644 is about 27.8684303527.

The spelling of 21644 in words is "twenty-one thousand, six hundred forty-four".