91991 has 4 divisors (see below), whose sum is σ = 93432. Its totient is φ = 90552.

The previous prime is 91969. The next prime is 91997. The reversal of 91991 is 19919.

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

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-91991 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 91991.

It is not an unprimeable number, because it can be changed into a prime (91997) 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, 620 + ... + 753.

It is an arithmetic number, because the mean of its divisors is an integer number (23358).

2⁹¹⁹⁹¹ is an apocalyptic number.

91991 is a deficient number, since it is larger than the sum of its proper divisors (1441).

91991 is a wasteful number, since it uses less digits than its factorization.

91991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1440.

The product of its digits is 729, while the sum is 29.

The square root of 91991 is about 303.3001813386. The cubic root of 91991 is about 45.1421022337.

The spelling of 91991 in words is "ninety-one thousand, nine hundred ninety-one".