991991891991 has 8 divisors (see below), whose sum is σ = 1324948156144. Its totient is φ = 660181777920.

The previous prime is 991991891977. The next prime is 991991891999. The reversal of 991991891991 is 199198199199.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 991991891991 - 2¹⁷ = 991991760919 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 (991991891999) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 286535500 + ... + 286538961.

It is an arithmetic number, because the mean of its divisors is an integer number (165618519518).

Almost surely, 2^991991891991 is an apocalyptic number.

991991891991 is a deficient number, since it is larger than the sum of its proper divisors (332956264153).

991991891991 is a wasteful number, since it uses less digits than its factorization.

991991891991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 573075041.

The product of its digits is 38263752, while the sum is 75.

The spelling of 991991891991 in words is "nine hundred ninety-one billion, nine hundred ninety-one million, eight hundred ninety-one thousand, nine hundred ninety-one".