21660 has 36 divisors (see below), whose sum is σ = 64008. Its totient is φ = 5472.

The previous prime is 21649. The next prime is 21661. The reversal of 21660 is 6612.

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

It is a nialpdrome in base 15.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21661) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 1131 + ... + 1149.

It is an arithmetic number, because the mean of its divisors is an integer number (1778).

2²¹⁶⁶⁰ is an apocalyptic number.

21660 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 21660, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32004).

21660 is an abundant number, since it is smaller than the sum of its proper divisors (42348).

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

21660 is a wasteful number, since it uses less digits than its factorization.

21660 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 50 (or 29 counting only the distinct ones).

The product of its (nonzero) digits is 72, while the sum is 15.

The square root of 21660 is about 147.1733671559. The cubic root of 21660 is about 27.8752957666.

Multiplying 21660 by its sum of digits (15), we get a square (324900 = 570²).

21660 divided by its sum of digits (15) gives a square (1444 = 38²).

It can be divided in two parts, 216 and 60, that added together give a triangular number (276 = T₂₃).

The spelling of 21660 in words is "twenty-one thousand, six hundred sixty".