90991 has 4 divisors (see below), whose sum is σ = 95800. Its totient is φ = 86184.

The previous prime is 90989. The next prime is 90997. The reversal of 90991 is 19909.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 19909 = 43 ⋅463.

It is not a de Polignac number, because 90991 - 2¹ = 90989 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (90997) 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, 2376 + ... + 2413.

It is an arithmetic number, because the mean of its divisors is an integer number (23950).

2⁹⁰⁹⁹¹ is an apocalyptic number.

90991 is a deficient number, since it is larger than the sum of its proper divisors (4809).

90991 is a wasteful number, since it uses less digits than its factorization.

90991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4808.

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

The square root of 90991 is about 301.6471448564. The cubic root of 90991 is about 44.9779315660.

Adding to 90991 its sum of digits (28), we get a palindrome (91019).

It can be divided in two parts, 909 and 91, that added together give a cube (1000 = 10³).

The spelling of 90991 in words is "ninety thousand, nine hundred ninety-one".