9991 has 4 divisors (see below), whose sum is σ = 10192. Its totient is φ = 9792.

The previous prime is 9973. The next prime is 10007. The reversal of 9991 is 1999.

9991 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

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 9991 - 2⁹ = 9479 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 10.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (9901) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 46 + ... + 148.

It is an arithmetic number, because the mean of its divisors is an integer number (2548).

2⁹⁹⁹¹ is an apocalyptic number.

9991 is a deficient number, since it is larger than the sum of its proper divisors (201).

9991 is a wasteful number, since it uses less digits than its factorization.

9991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 200.

The product of its digits is 729, while the sum is 28.

The square root of 9991 is about 99.9549898704. The cubic root of 9991 is about 21.5378816563.

It can be divided in two parts, 999 and 1, that added together give a cube (1000 = 10³).

The spelling of 9991 in words is "nine thousand, nine hundred ninety-one".