21866 has 12 divisors (see below), whose sum is σ = 36582. Its totient is φ = 9744.

The previous prime is 21863. The next prime is 21871. The reversal of 21866 is 66812.

It can be written as a sum of positive squares in 3 ways, for example, as 15625 + 6241 = 125^2 + 79^2 .

It is a plaindrome in base 4 and base 16.

It is a zygodrome in base 4.

It is not an unprimeable number, because it can be changed into a prime (21863) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 740 + ... + 768.

2²¹⁸⁶⁶ is an apocalyptic number.

21866 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

21866 is a deficient number, since it is larger than the sum of its proper divisors (14716).

21866 is a wasteful number, since it uses less digits than its factorization.

21866 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 73 (or 44 counting only the distinct ones).

The product of its digits is 576, while the sum is 23.

The square root of 21866 is about 147.8715658942. The cubic root of 21866 is about 27.9633875210.

The spelling of 21866 in words is "twenty-one thousand, eight hundred sixty-six".