18991 has 4 divisors (see below), whose sum is σ = 21712. Its totient is φ = 16272.

The previous prime is 18979. The next prime is 19001. The reversal of 18991 is 19981.

18991 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

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 18991 - 2⁵ = 18959 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 (18911) 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, 1350 + ... + 1363.

It is an arithmetic number, because the mean of its divisors is an integer number (5428).

2¹⁸⁹⁹¹ is an apocalyptic number.

18991 is a deficient number, since it is larger than the sum of its proper divisors (2721).

18991 is an equidigital number, since it uses as much as digits as its factorization.

18991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2720.

The product of its digits is 648, while the sum is 28.

The square root of 18991 is about 137.8078372227. The cubic root of 18991 is about 26.6798025560.

Subtracting 18991 from its reverse (19981), we obtain a triangular number (990 = T₄₄).

The spelling of 18991 in words is "eighteen thousand, nine hundred ninety-one".