21806 has 4 divisors (see below), whose sum is σ = 32712. Its totient is φ = 10902.

The previous prime is 21803. The next prime is 21817. The reversal of 21806 is 60812.

It is a semiprime because it is the product of two primes.

It is a Curzon number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 21806.

It is not an unprimeable number, because it can be changed into a prime (21803) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5450 + ... + 5453.

It is an arithmetic number, because the mean of its divisors is an integer number (8178).

2²¹⁸⁰⁶ is an apocalyptic number.

21806 is a deficient number, since it is larger than the sum of its proper divisors (10906).

21806 is a wasteful number, since it uses less digits than its factorization.

21806 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10905.

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

The square root of 21806 is about 147.6685477683. The cubic root of 21806 is about 27.9377870420.

The spelling of 21806 in words is "twenty-one thousand, eight hundred six".