21811 has 4 divisors (see below), whose sum is σ = 23112. Its totient is φ = 20512.

The previous prime is 21803. The next prime is 21817. The reversal of 21811 is 11812.

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

It is a cyclic number.

It is not a de Polignac number, because 21811 - 2³ = 21803 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 16.

It is a zygodrome in base 16.

It is a junction number, because it is equal to n+sod(n) for n = 21791 and 21800.

It is not an unprimeable number, because it can be changed into a prime (21817) 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, 625 + ... + 658.

It is an arithmetic number, because the mean of its divisors is an integer number (5778).

2²¹⁸¹¹ is an apocalyptic number.

21811 is a deficient number, since it is larger than the sum of its proper divisors (1301).

21811 is a wasteful number, since it uses less digits than its factorization.

21811 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1300.

The product of its digits is 16, while the sum is 13.

The square root of 21811 is about 147.6854766048. The cubic root of 21811 is about 27.9399222076.

Subtracting from 21811 its reverse (11812), we obtain a palindrome (9999).

The spelling of 21811 in words is "twenty-one thousand, eight hundred eleven".