1991082991 has 4 divisors (see below), whose sum is σ = 2013454800. Its totient is φ = 1968711184.

The previous prime is 1991082967. The next prime is 1991082997. The reversal of 1991082991 is 1992801991.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 1992801991 = 571 ⋅3490021.

It is a cyclic number.

It is not a de Polignac number, because 1991082991 - 2¹¹ = 1991080943 is a prime.

It is a super-2 number, since 2×1991082991² = 7928822954099012162, which contains 22 as substring.

It is a Smith number, since the sum of its digits (49) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (1991082997) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 11185771 + ... + 11185948.

It is an arithmetic number, because the mean of its divisors is an integer number (503363700).

Almost surely, 2^1991082991 is an apocalyptic number.

1991082991 is a deficient number, since it is larger than the sum of its proper divisors (22371809).

1991082991 is an equidigital number, since it uses as much as digits as its factorization.

1991082991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 22371808.

The product of its (nonzero) digits is 104976, while the sum is 49.

The square root of 1991082991 is about 44621.5529873177. The cubic root of 1991082991 is about 1258.0458056423.

The spelling of 1991082991 in words is "one billion, nine hundred ninety-one million, eighty-two thousand, nine hundred ninety-one".