81809 has 16 divisors (see below), whose sum is σ = 107520. Its totient is φ = 60480.

The previous prime is 81799. The next prime is 81817. The reversal of 81809 is 90818.

It is not a de Polignac number, because 81809 - 2⁸ = 81553 is a prime.

It is a Smith number, since the sum of its digits (26) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is not an unprimeable number, because it can be changed into a prime (81839) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 2624 + ... + 2654.

It is an arithmetic number, because the mean of its divisors is an integer number (6720).

2⁸¹⁸⁰⁹ is an apocalyptic number.

It is an amenable number.

81809 is a deficient number, since it is larger than the sum of its proper divisors (25711).

81809 is a wasteful number, since it uses less digits than its factorization.

81809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 80.

The product of its (nonzero) digits is 576, while the sum is 26.

The square root of 81809 is about 286.0227263698. The cubic root of 81809 is about 43.4110570906.

Subtracting 81809 from its reverse (90818), we obtain a palindrome (9009).

The spelling of 81809 in words is "eighty-one thousand, eight hundred nine".