100991 has 4 divisors (see below), whose sum is σ = 110184. Its totient is φ = 91800.

The previous prime is 100987. The next prime is 100999. The reversal of 100991 is 199001.

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 100991 - 2² = 100987 is a prime.

It is an Ulam 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 (100999) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 4580 + ... + 4601.

It is an arithmetic number, because the mean of its divisors is an integer number (27546).

2¹⁰⁰⁹⁹¹ is an apocalyptic number.

100991 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

100991 is a deficient number, since it is larger than the sum of its proper divisors (9193).

100991 is an equidigital number, since it uses as much as digits as its factorization.

100991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9192.

The product of its (nonzero) digits is 81, while the sum is 20.

The square root of 100991 is about 317.7908116985. The cubic root of 100991 is about 46.5687117668.

Adding to 100991 its reverse (199001), we get a palindrome (299992).

It can be divided in two parts, 1009 and 91, that multiplied together give a palindrome (91819).

The spelling of 100991 in words is "one hundred thousand, nine hundred ninety-one".