991235661091 has 4 divisors (see below), whose sum is σ = 1067484558112. Its totient is φ = 914986764072.

The previous prime is 991235661029. The next prime is 991235661161. The reversal of 991235661091 is 190166532199.

It is a happy number.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-991235661091 is a prime.

It is a super-2 number, since 2×991235661091² (a number of 25 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (991235661001) 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, 38124448491 + ... + 38124448516.

It is an arithmetic number, because the mean of its divisors is an integer number (266871139528).

Almost surely, 2^991235661091 is an apocalyptic number.

991235661091 is a deficient number, since it is larger than the sum of its proper divisors (76248897021).

991235661091 is a wasteful number, since it uses less digits than its factorization.

991235661091 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 76248897020.

The product of its (nonzero) digits is 787320, while the sum is 52.

The spelling of 991235661091 in words is "nine hundred ninety-one billion, two hundred thirty-five million, six hundred sixty-one thousand, ninety-one".