11991 has 8 divisors (see below), whose sum is σ = 18304. Its totient is φ = 6840.

The previous prime is 11987. The next prime is 12007. The reversal of 11991 is 19911.

11991 = T₅₅ + T₅₆ + ... + T₆₁.

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

It is not a de Polignac number, because 11991 - 2² = 11987 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (21), and also a Moran number because the ratio is a prime number: 571 = 11991 / (1 + 1 + 9 + 9 + 1).

It is the 110-th Hogben number.

It is a Duffinian number.

11991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (11941) by changing a digit.

It is a nontrivial repunit in base 109.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 265 + ... + 306.

It is an arithmetic number, because the mean of its divisors is an integer number (2288).

2¹¹⁹⁹¹ is an apocalyptic number.

11991 is a deficient number, since it is larger than the sum of its proper divisors (6313).

11991 is an equidigital number, since it uses as much as digits as its factorization.

11991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 581.

The product of its digits is 81, while the sum is 21.

The square root of 11991 is about 109.5034246040. The cubic root of 11991 is about 22.8885598484. Note that the first 3 decimals are identical.

It can be divided in two parts, 119 and 91, that added together give a triangular number (210 = T₂₀).

The spelling of 11991 in words is "eleven thousand, nine hundred ninety-one".