991081981991 has 4 divisors (see below), whose sum is σ = 1000894674984. Its totient is φ = 981269289000.

The previous prime is 991081981951. The next prime is 991081981997. The reversal of 991081981991 is 199189180199.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199189180199 = 191 ⋅1042875289.

It is a cyclic number.

It is not a de Polignac number, because 991081981991 - 2¹⁴ = 991081965607 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (991081981997) 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 3 ways as a sum of consecutive naturals, for example, 4906346345 + ... + 4906346546.

It is an arithmetic number, because the mean of its divisors is an integer number (250223668746).

Almost surely, 2^991081981991 is an apocalyptic number.

991081981991 is a deficient number, since it is larger than the sum of its proper divisors (9812692993).

991081981991 is a wasteful number, since it uses less digits than its factorization.

991081981991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9812692992.

The product of its (nonzero) digits is 3779136, while the sum is 65.

The spelling of 991081981991 in words is "nine hundred ninety-one billion, eighty-one million, nine hundred eighty-one thousand, nine hundred ninety-one".