991 has 2 divisors, whose sum is σ = 992.
Its totient is φ = 990.
The previous prime is 983. The next prime is 997. The reversal of 991 is 199.
991 is nontrivially palindromic in base 4.
It is a strong prime.
It is an emirp because it is prime and its reverse (199) is a distict prime.
It is a cyclic number.
It is not a de Polignac number, because 991 - 23 = 983 is a prime.
It is a Chen prime.
It is an Ulam number.
It is a pancake number, because a pancake can be divided into 991 parts by 44 straight cuts.
991 is a lucky number.
It is a plaindrome in base 16.
It is a nialpdrome in base 6, base 10 and base 11.
It is a congruent number.
It is not a weakly prime, because it can be changed into another prime (997) by changing a digit.
It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495 + 496.
It is an arithmetic number, because the mean of its divisors is an integer number (496).
991 is a deficient number, since it is larger than the sum of its proper divisors (1).
991 is an equidigital number, since it uses as much as digits as its factorization.
991 is an odious number, because the sum of its binary digits is odd.
The product of its digits is 81, while the sum is 19.
The square root of 991 is about 31.4801524774.
The cubic root of 991 is about 9.9699095473.
It can be divided in two parts, 99 and 1, that added together give a square (100 = 102).
The spelling of 991 in words is "nine hundred ninety-one", and thus it is an aban number.