187991 has 4 divisors (see below), whose sum is σ = 191592. Its totient is φ = 184392.

The previous prime is 187987. The next prime is 188011. The reversal of 187991 is 199781.

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 187991 - 2² = 187987 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 (187921) 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, 1721 + ... + 1826.

It is an arithmetic number, because the mean of its divisors is an integer number (47898).

2¹⁸⁷⁹⁹¹ is an apocalyptic number.

187991 is a deficient number, since it is larger than the sum of its proper divisors (3601).

187991 is an equidigital number, since it uses as much as digits as its factorization.

187991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3600.

The product of its digits is 4536, while the sum is 35.

The square root of 187991 is about 433.5792891733. The cubic root of 187991 is about 57.2856289983.

The spelling of 187991 in words is "one hundred eighty-seven thousand, nine hundred ninety-one".