18321991 has 2 divisors, whose sum is σ = 18321992. Its totient is φ = 18321990.

The previous prime is 18321977. The next prime is 18322013. The reversal of 18321991 is 19912381.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 18321991 - 2¹⁷ = 18190919 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (18321091) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 9160995 + 9160996.

It is an arithmetic number, because the mean of its divisors is an integer number (9160996).

Almost surely, 2^18321991 is an apocalyptic number.

18321991 is a deficient number, since it is larger than the sum of its proper divisors (1).

18321991 is an equidigital number, since it uses as much as digits as its factorization.

18321991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 3888, while the sum is 34.

The square root of 18321991 is about 4280.4194887885. The cubic root of 18321991 is about 263.6276076110.

The spelling of 18321991 in words is "eighteen million, three hundred twenty-one thousand, nine hundred ninety-one".