21666 has 16 divisors (see below), whose sum is σ = 45504. Its totient is φ = 6864.

The previous prime is 21661. The next prime is 21673. The reversal of 21666 is 66612.

It is a nude number because it is divisible by every one of its digits.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21666.

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

21666 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 60 + ... + 216.

It is an arithmetic number, because the mean of its divisors is an integer number (2844).

2²¹⁶⁶⁶ is an apocalyptic number.

21666 is an abundant number, since it is smaller than the sum of its proper divisors (23838).

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 (22752).

21666 is a wasteful number, since it uses less digits than its factorization.

21666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 185.

The product of its digits is 432, while the sum is 21.

The square root of 21666 is about 147.1937498673. The cubic root of 21666 is about 27.8778694252.

It can be divided in two parts, 2166 and 6, that multiplied together give a square (12996 = 114²).

The spelling of 21666 in words is "twenty-one thousand, six hundred sixty-six".