191991 has 4 divisors (see below), whose sum is σ = 255992. Its totient is φ = 127992.

The previous prime is 191977. The next prime is 191999. The reversal of 191991 is 199191.

It is a semiprime because it is the product of two primes.

It is not a de Polignac number, because 191991 - 2¹⁵ = 159223 is a prime.

It is a D-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 (191999) 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, 31996 + ... + 32001.

It is an arithmetic number, because the mean of its divisors is an integer number (63998).

2¹⁹¹⁹⁹¹ is an apocalyptic number.

191991 is a deficient number, since it is larger than the sum of its proper divisors (64001).

191991 is an equidigital number, since it uses as much as digits as its factorization.

191991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 64000.

The product of its digits is 729, while the sum is 30.

The square root of 191991 is about 438.1677760858. The cubic root of 191991 is about 57.6890813922.

The spelling of 191991 in words is "one hundred ninety-one thousand, nine hundred ninety-one".