181991 has 4 divisors (see below), whose sum is σ = 183552. Its totient is φ = 180432.

The previous prime is 181981. The next prime is 181997. The reversal of 181991 is 199181.

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 181991 - 2⁶ = 181927 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (181997) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 590 + ... + 843.

It is an arithmetic number, because the mean of its divisors is an integer number (45888).

2¹⁸¹⁹⁹¹ is an apocalyptic number.

181991 is a deficient number, since it is larger than the sum of its proper divisors (1561).

181991 is a wasteful number, since it uses less digits than its factorization.

181991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1560.

The product of its digits is 648, while the sum is 29.

The square root of 181991 is about 426.6040318609. The cubic root of 181991 is about 56.6695769363.

The spelling of 181991 in words is "one hundred eighty-one thousand, nine hundred ninety-one".