20811 has 8 divisors (see below), whose sum is σ = 31744. Its totient is φ = 11880.

The previous prime is 20809. The next prime is 20849. The reversal of 20811 is 11802.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 20811 - 2¹ = 20809 is a prime.

It is a Duffinian number.

20811 is a lucky number.

It is a plaindrome in base 11.

It is not an unprimeable number, because it can be changed into a prime (20011) 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 7 ways as a sum of consecutive naturals, for example, 475 + ... + 516.

It is an arithmetic number, because the mean of its divisors is an integer number (3968).

2²⁰⁸¹¹ is an apocalyptic number.

20811 is a gapful number since it is divisible by the number (21) formed by its first and last digit.

20811 is a deficient number, since it is larger than the sum of its proper divisors (10933).

20811 is an equidigital number, since it uses as much as digits as its factorization.

20811 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1001.

The product of its (nonzero) digits is 16, while the sum is 12.

The square root of 20811 is about 144.2601816164. The cubic root of 20811 is about 27.5062244863.

Subtracting from 20811 its reverse (11802), we obtain a palindrome (9009).

The spelling of 20811 in words is "twenty thousand, eight hundred eleven".