101991 has 4 divisors (see below), whose sum is σ = 135992. Its totient is φ = 67992.

The previous prime is 101987. The next prime is 101999. The reversal of 101991 is 199101.

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

It is not a de Polignac number, because 101991 - 2² = 101987 is a prime.

It is a D-number.

It is a Duffinian number.

It is a zygodrome in base 2.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (101999) 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, 16996 + ... + 17001.

It is an arithmetic number, because the mean of its divisors is an integer number (33998).

2¹⁰¹⁹⁹¹ is an apocalyptic number.

101991 is a deficient number, since it is larger than the sum of its proper divisors (34001).

101991 is an equidigital number, since it uses as much as digits as its factorization.

101991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 34000.

The product of its (nonzero) digits is 81, while the sum is 21.

The square root of 101991 is about 319.3602980961. The cubic root of 101991 is about 46.7219130288.

It can be divided in two parts, 1019 and 91, that multiplied together give a palindrome (92729).

The spelling of 101991 in words is "one hundred one thousand, nine hundred ninety-one".