99991 has 2 divisors, whose sum is σ = 99992. Its totient is φ = 99990.

The previous prime is 99989. The next prime is 100003. The reversal of 99991 is 19999.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 99991 - 2¹ = 99989 is a prime.

Together with 99989, it forms a pair of twin primes.

It is a Chen prime.

It is a plaindrome in base 13.

It is a nialpdrome in base 10.

It is a self number, because there is not a number n which added to its sum of digits gives 99991.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (99901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 49995 + 49996.

It is an arithmetic number, because the mean of its divisors is an integer number (49996).

2⁹⁹⁹⁹¹ is an apocalyptic number.

99991 is a deficient number, since it is larger than the sum of its proper divisors (1).

99991 is an equidigital number, since it uses as much as digits as its factorization.

99991 is an odious number, because the sum of its binary digits is odd.

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

The square root of 99991 is about 316.2135354472. The cubic root of 99991 is about 46.4144958177.

It can be divided in two parts, 9999 and 1, that added together give a 4-th power (10000 = 10⁴).

The spelling of 99991 in words is "ninety-nine thousand, nine hundred ninety-one".