991908991 has 2 divisors, whose sum is σ = 991908992. Its totient is φ = 991908990.

The previous prime is 991908949. The next prime is 991909021. The reversal of 991908991 is 199809199.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 991908991 - 2⁹ = 991908479 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (991908901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495954495 + 495954496.

It is an arithmetic number, because the mean of its divisors is an integer number (495954496).

Almost surely, 2^991908991 is an apocalyptic number.

991908991 is a deficient number, since it is larger than the sum of its proper divisors (1).

991908991 is an equidigital number, since it uses as much as digits as its factorization.

991908991 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 472392, while the sum is 55.

The square root of 991908991 is about 31494.5866935891. The cubic root of 991908991 is about 997.2956903015.

The spelling of 991908991 in words is "nine hundred ninety-one million, nine hundred eight thousand, nine hundred ninety-one".