111991 has 4 divisors (see below), whose sum is σ = 122184. Its totient is φ = 101800.

The previous prime is 111977. The next prime is 111997. The reversal of 111991 is 199111.

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 111991 - 2⁵ = 111959 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 (111997) 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, 5080 + ... + 5101.

It is an arithmetic number, because the mean of its divisors is an integer number (30546).

2¹¹¹⁹⁹¹ is an apocalyptic number.

111991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

111991 is a deficient number, since it is larger than the sum of its proper divisors (10193).

111991 is a wasteful number, since it uses less digits than its factorization.

111991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10192.

The product of its digits is 81, while the sum is 22.

The square root of 111991 is about 334.6505640216. The cubic root of 111991 is about 48.2015541013.

It can be divided in two parts, 11 and 1991, that added together give a palindrome (2002).

The spelling of 111991 in words is "one hundred eleven thousand, nine hundred ninety-one".