991809 has 36 divisors (see below), whose sum is σ = 1805076. Its totient is φ = 520128.

The previous prime is 991777. The next prime is 991811. The reversal of 991809 is 908199.

It can be written as a sum of positive squares in 2 ways, for example, as 540225 + 451584 = 735^2 + 672^2 .

It is not a de Polignac number, because 991809 - 2⁵ = 991777 is a prime.

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 991809.

It is not an unprimeable number, because it can be changed into a prime (991889) by changing a digit.

It is a polite number, since it can be written in 35 ways as a sum of consecutive naturals, for example, 5647 + ... + 5819.

It is an arithmetic number, because the mean of its divisors is an integer number (50141).

2⁹⁹¹⁸⁰⁹ is an apocalyptic number.

It is an amenable number.

991809 is a deficient number, since it is larger than the sum of its proper divisors (813267).

991809 is a wasteful number, since it uses less digits than its factorization.

991809 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 206 (or 196 counting only the distinct ones).

The product of its (nonzero) digits is 5832, while the sum is 36.

The square root of 991809 is about 995.8960789159. The cubic root of 991809 is about 99.7262177837.

It can be divided in two parts, 9 and 91809, that multiplied together give a square (826281 = 909²).

The spelling of 991809 in words is "nine hundred ninety-one thousand, eight hundred nine".