20809 has 2 divisors, whose sum is σ = 20810. Its totient is φ = 20808.

The previous prime is 20807. The next prime is 20849. The reversal of 20809 is 90802.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15625 + 5184 = 125^2 + 72^2 .

It is a cyclic number.

It is not a de Polignac number, because 20809 - 2¹ = 20807 is a prime.

Together with 20807, it forms a pair of twin primes.

It is a plaindrome in base 7.

It is not a weakly prime, because it can be changed into another prime (20807) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10404 + 10405.

It is an arithmetic number, because the mean of its divisors is an integer number (10405).

2²⁰⁸⁰⁹ is an apocalyptic number.

It is an amenable number.

20809 is a deficient number, since it is larger than the sum of its proper divisors (1).

20809 is an equidigital number, since it uses as much as digits as its factorization.

20809 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 144, while the sum is 19.

The square root of 20809 is about 144.2532495301. The cubic root of 20809 is about 27.5053433143.

The spelling of 20809 in words is "twenty thousand, eight hundred nine".