91999 has 4 divisors (see below), whose sum is σ = 92664. Its totient is φ = 91336.

The previous prime is 91997. The next prime is 92003. The reversal of 91999 is 99919.

91999 is nontrivially palindromic in base 11.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length, and also an emirpimes, since its reverse is a distinct semiprime: 99919 = 163 ⋅613.

It is a cyclic number.

It is not a de Polignac number, because 91999 - 2¹ = 91997 is a prime.

It is an Ulam number.

It is a Duffinian number.

91999 is a modest number, since divided by 999 gives 91 as remainder.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (91997) 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, 37 + ... + 430.

It is an arithmetic number, because the mean of its divisors is an integer number (23166).

2⁹¹⁹⁹⁹ is an apocalyptic number.

91999 is a deficient number, since it is larger than the sum of its proper divisors (665).

91999 is a wasteful number, since it uses less digits than its factorization.

91999 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 664.

The product of its digits is 6561, while the sum is 37.

The square root of 91999 is about 303.3133693064. The cubic root of 91999 is about 45.1434107905.

It can be divided in two parts, 91 and 999, that multiplied together give a palindrome (90909).

The spelling of 91999 in words is "ninety-one thousand, nine hundred ninety-nine".