21728 has 24 divisors (see below), whose sum is σ = 49392. Its totient is φ = 9216.

The previous prime is 21727. The next prime is 21737. The reversal of 21728 is 82712.

It is a nude number because it is divisible by every one of its digits and also a Zuckerman number because it is divisible by the product of its digits.

It is a plaindrome in base 15.

It is a zygodrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21727) 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, 176 + ... + 272.

It is an arithmetic number, because the mean of its divisors is an integer number (2058).

2²¹⁷²⁸ is an apocalyptic number.

21728 is a gapful number since it is divisible by the number (28) 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 21728, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (24696).

21728 is an abundant number, since it is smaller than the sum of its proper divisors (27664).

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

21728 is an equidigital number, since it uses as much as digits as its factorization.

21728 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 114 (or 106 counting only the distinct ones).

The product of its digits is 224, while the sum is 20.

The square root of 21728 is about 147.4042061815. The cubic root of 21728 is about 27.9044361167.

Adding to 21728 its product of digits (224), we get a cube (21952 = 28³).

The spelling of 21728 in words is "twenty-one thousand, seven hundred twenty-eight".