21680 has 20 divisors (see below), whose sum is σ = 50592. Its totient is φ = 8640.

The previous prime is 21673. The next prime is 21683. The reversal of 21680 is 8612.

It is a tau number, because it is divible by the number of its divisors (20).

It is a hoax number, since the sum of its digits (17) coincides with the sum of the digits of its distinct prime factors.

It is a nialpdrome in base 15.

It is a zygodrome in base 15.

It is not an unprimeable number, because it can be changed into a prime (21683) 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, 56 + ... + 215.

2²¹⁶⁸⁰ is an apocalyptic number.

21680 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21680 is an abundant number, since it is smaller than the sum of its proper divisors (28912).

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 (25296).

21680 is a wasteful number, since it uses less digits than its factorization.

21680 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 284 (or 278 counting only the distinct ones).

The product of its (nonzero) digits is 96, while the sum is 17.

The square root of 21680 is about 147.2412985544. The cubic root of 21680 is about 27.8838727812.

The spelling of 21680 in words is "twenty-one thousand, six hundred eighty".