171991 has 4 divisors (see below), whose sum is σ = 172872. Its totient is φ = 171112.

The previous prime is 171947. The next prime is 172001. The reversal of 171991 is 199171.

171991 is a nontrivial binomial coefficient, being equal to C(587, 2).

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is not a de Polignac number, because 171991 - 2⁷ = 171863 is a prime.

It is a Duffinian number.

171991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (171091) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 1 + ... + 586.

It is an arithmetic number, because the mean of its divisors is an integer number (43218).

2¹⁷¹⁹⁹¹ is an apocalyptic number.

171991 is the 586-th triangular number.

171991 is the 196-th centered nonagonal number.

171991 is a deficient number, since it is larger than the sum of its proper divisors (881).

171991 is an equidigital number, since it uses as much as digits as its factorization.

171991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 880.

The product of its digits is 567, while the sum is 28.

The square root of 171991 is about 414.7179764611. The cubic root of 171991 is about 55.6120076545.

The spelling of 171991 in words is "one hundred seventy-one thousand, nine hundred ninety-one".