811981 has 2 divisors, whose sum is σ = 811982. Its totient is φ = 811980.

The previous prime is 811961. The next prime is 811991. The reversal of 811981 is 189118.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 792100 + 19881 = 890^2 + 141^2 .

It is a cyclic number.

It is not a de Polignac number, because 811981 - 2⁹ = 811469 is a prime.

It is a super-2 number, since 2×811981² = 1318626288722, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (811931) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 405990 + 405991.

It is an arithmetic number, because the mean of its divisors is an integer number (405991).

2⁸¹¹⁹⁸¹ is an apocalyptic number.

It is an amenable number.

811981 is a deficient number, since it is larger than the sum of its proper divisors (1).

811981 is an equidigital number, since it uses as much as digits as its factorization.

811981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 576, while the sum is 28.

The square root of 811981 is about 901.0998834757. The cubic root of 811981 is about 93.2929062445.

It can be divided in two parts, 81 and 1981, that multiplied together give a triangular number (160461 = T₅₆₆).

The spelling of 811981 in words is "eight hundred eleven thousand, nine hundred eighty-one".