18091991 has 2 divisors, whose sum is σ = 18091992. Its totient is φ = 18091990.

The previous prime is 18091979. The next prime is 18091993. The reversal of 18091991 is 19919081.

It is a happy number.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-18091991 is a prime.

Together with 18091993, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18091993) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9045995 + 9045996.

It is an arithmetic number, because the mean of its divisors is an integer number (9045996).

Almost surely, 2^18091991 is an apocalyptic number.

18091991 is a deficient number, since it is larger than the sum of its proper divisors (1).

18091991 is an equidigital number, since it uses as much as digits as its factorization.

18091991 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 5832, while the sum is 38.

The square root of 18091991 is about 4253.4681143744. The cubic root of 18091991 is about 262.5198340267.

The spelling of 18091991 in words is "eighteen million, ninety-one thousand, nine hundred ninety-one".