18991891 has 2 divisors, whose sum is σ = 18991892. Its totient is φ = 18991890.

The previous prime is 18991879. The next prime is 18991909. The reversal of 18991891 is 19819981.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 18991891 - 2¹⁵ = 18959123 is a prime.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (18991831) 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 as a sum of consecutive naturals, namely, 9495945 + 9495946.

It is an arithmetic number, because the mean of its divisors is an integer number (9495946).

Almost surely, 2^18991891 is an apocalyptic number.

18991891 is a deficient number, since it is larger than the sum of its proper divisors (1).

18991891 is an equidigital number, since it uses as much as digits as its factorization.

18991891 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 46656, while the sum is 46.

The square root of 18991891 is about 4357.9686781802. The cubic root of 18991891 is about 266.8021979459.

The spelling of 18991891 in words is "eighteen million, nine hundred ninety-one thousand, eight hundred ninety-one".