991109891 has 2 divisors, whose sum is σ = 991109892. Its totient is φ = 991109890.

The previous prime is 991109879. The next prime is 991109909. The reversal of 991109891 is 198901199.

It is a happy number.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 991109891 - 2¹⁸ = 990847747 is a prime.

It is not a weakly prime, because it can be changed into another prime (991109291) 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, 495554945 + 495554946.

It is an arithmetic number, because the mean of its divisors is an integer number (495554946).

Almost surely, 2^991109891 is an apocalyptic number.

991109891 is a deficient number, since it is larger than the sum of its proper divisors (1).

991109891 is an equidigital number, since it uses as much as digits as its factorization.

991109891 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 52488, while the sum is 47.

The square root of 991109891 is about 31481.8978303405. The cubic root of 991109891 is about 997.0278051431.

The spelling of 991109891 in words is "nine hundred ninety-one million, one hundred nine thousand, eight hundred ninety-one".