819991 has 2 divisors, whose sum is σ = 819992. Its totient is φ = 819990.

The previous prime is 819989. The next prime is 820037. The reversal of 819991 is 199918.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 819991 - 2¹ = 819989 is a prime.

Together with 819989, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 819991.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (819911) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 409995 + 409996.

It is an arithmetic number, because the mean of its divisors is an integer number (409996).

2⁸¹⁹⁹⁹¹ is an apocalyptic number.

819991 is a deficient number, since it is larger than the sum of its proper divisors (1).

819991 is an equidigital number, since it uses as much as digits as its factorization.

819991 is an odious number, because the sum of its binary digits is odd.

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

The square root of 819991 is about 905.5335443814. The cubic root of 819991 is about 93.5986737947.

Adding to 819991 its sum of digits (37), we get a palindrome (820028).

The spelling of 819991 in words is "eight hundred nineteen thousand, nine hundred ninety-one".