991891 has 4 divisors (see below), whose sum is σ = 994000. Its totient is φ = 989784.

The previous prime is 991889. The next prime is 991901. The reversal of 991891 is 198199.

991891 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, and also an emirpimes, since its reverse is a distinct semiprime: 198199 = 47 ⋅4217.

It is a cyclic number.

It is not a de Polignac number, because 991891 - 2¹ = 991889 is a prime.

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 991891.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (991811) 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, 10 + ... + 1408.

It is an arithmetic number, because the mean of its divisors is an integer number (248500).

2⁹⁹¹⁸⁹¹ is an apocalyptic number.

991891 is a deficient number, since it is larger than the sum of its proper divisors (2109).

991891 is a wasteful number, since it uses less digits than its factorization.

991891 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2108.

The product of its digits is 5832, while the sum is 37.

The square root of 991891 is about 995.9372470191. The cubic root of 991891 is about 99.7289660698.

The spelling of 991891 in words is "nine hundred ninety-one thousand, eight hundred ninety-one".