811991 has 2 divisors, whose sum is σ = 811992. Its totient is φ = 811990.

The previous prime is 811981. The next prime is 811997. The reversal of 811991 is 199118.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 811991 - 2¹⁰ = 810967 is a prime.

It is a plaindrome in base 13.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (811997) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 405995 + 405996.

It is an arithmetic number, because the mean of its divisors is an integer number (405996).

2⁸¹¹⁹⁹¹ is an apocalyptic number.

811991 is a deficient number, since it is larger than the sum of its proper divisors (1).

811991 is an equidigital number, since it uses as much as digits as its factorization.

811991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 648, while the sum is 29.

The square root of 811991 is about 901.1054322331. The cubic root of 811991 is about 93.2932892277.

The spelling of 811991 in words is "eight hundred eleven thousand, nine hundred ninety-one".