131991 has 4 divisors (see below), whose sum is σ = 175992. Its totient is φ = 87992.

The previous prime is 131969. The next prime is 132001. The reversal of 131991 is 199131.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 199131 = 3 ⋅66377.

It is a cyclic number.

It is not a de Polignac number, because 131991 - 2⁵ = 131959 is a prime.

It is a D-number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (131941) 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, 21996 + ... + 22001.

It is an arithmetic number, because the mean of its divisors is an integer number (43998).

2¹³¹⁹⁹¹ is an apocalyptic number.

131991 is a deficient number, since it is larger than the sum of its proper divisors (44001).

131991 is an equidigital number, since it uses as much as digits as its factorization.

131991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 44000.

The product of its digits is 243, while the sum is 24.

The square root of 131991 is about 363.3056564382. The cubic root of 131991 is about 50.9152764786.

The spelling of 131991 in words is "one hundred thirty-one thousand, nine hundred ninety-one".