115991 has 4 divisors (see below), whose sum is σ = 122832. Its totient is φ = 109152.

The previous prime is 115987. The next prime is 116009. The reversal of 115991 is 199511.

It is a happy number.

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

It is a cyclic number.

It is not a de Polignac number, because 115991 - 2² = 115987 is a prime.

It is a Duffinian number.

It is a plaindrome in base 15.

It is a nialpdrome in base 7.

It is a self number, because there is not a number n which added to its sum of digits gives 115991.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (115901) 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, 3395 + ... + 3428.

It is an arithmetic number, because the mean of its divisors is an integer number (30708).

2¹¹⁵⁹⁹¹ is an apocalyptic number.

115991 is a deficient number, since it is larger than the sum of its proper divisors (6841).

115991 is an equidigital number, since it uses as much as digits as its factorization.

115991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6840.

The product of its digits is 405, while the sum is 26.

The square root of 115991 is about 340.5745146073. The cubic root of 115991 is about 48.7687282853.

The spelling of 115991 in words is "one hundred fifteen thousand, nine hundred ninety-one".