81991912791 has 32 divisors (see below), whose sum is σ = 115600782528. Its totient is φ = 51622824960.

The previous prime is 81991912777. The next prime is 81991912811. The reversal of 81991912791 is 19721919918.

It is not a de Polignac number, because 81991912791 - 2¹⁸ = 81991650647 is a prime.

It is a Smith number, since the sum of its digits (57) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81991912091) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 850821 + ... + 942273.

It is an arithmetic number, because the mean of its divisors is an integer number (3612524454).

Almost surely, 2^81991912791 is an apocalyptic number.

81991912791 is a deficient number, since it is larger than the sum of its proper divisors (33608869737).

81991912791 is a wasteful number, since it uses less digits than its factorization.

81991912791 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 91731.

The product of its digits is 734832, while the sum is 57.

The spelling of 81991912791 in words is "eighty-one billion, nine hundred ninety-one million, nine hundred twelve thousand, seven hundred ninety-one".