1991809 has 4 divisors (see below), whose sum is σ = 2003904. Its totient is φ = 1979716.

The previous prime is 1991797. The next prime is 1991819. The reversal of 1991809 is 9081991.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 1991809 - 2⁷ = 1991681 is a prime.

It is a Duffinian number.

1991809 is a lucky number.

It is not an unprimeable number, because it can be changed into a prime (1991819) 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, 5797 + ... + 6130.

It is an arithmetic number, because the mean of its divisors is an integer number (500976).

2^1991809 is an apocalyptic number.

It is an amenable number.

1991809 is a deficient number, since it is larger than the sum of its proper divisors (12095).

1991809 is a wasteful number, since it uses less digits than its factorization.

1991809 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 12094.

The product of its (nonzero) digits is 5832, while the sum is 37.

The square root of 1991809 is about 1411.3146353666. The cubic root of 1991809 is about 125.8198694228.

The spelling of 1991809 in words is "one million, nine hundred ninety-one thousand, eight hundred nine".