8991991 has 2 divisors, whose sum is σ = 8991992. Its totient is φ = 8991990.

The previous prime is 8991989. The next prime is 8992007. The reversal of 8991991 is 1991998.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 8991991 - 2¹ = 8991989 is a prime.

Together with 8991989, it forms a pair of twin primes.

8991991 is a lucky number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (8991901) 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, 4495995 + 4495996.

It is an arithmetic number, because the mean of its divisors is an integer number (4495996).

Almost surely, 2^8991991 is an apocalyptic number.

8991991 is a deficient number, since it is larger than the sum of its proper divisors (1).

8991991 is an equidigital number, since it uses as much as digits as its factorization.

8991991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 52488, while the sum is 46.

The square root of 8991991 is about 2998.6648695711. The cubic root of 8991991 is about 207.9466625443.

The spelling of 8991991 in words is "eight million, nine hundred ninety-one thousand, nine hundred ninety-one".