9999999991 has 12 divisors (see below), whose sum is σ = 10592223672. Its totient is φ = 9439388784.

The previous prime is 9999999967. The next prime is 10000000019. The reversal of 9999999991 is 1999999999.

It is not a de Polignac number, because 9999999991 - 2⁹ = 9999999479 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 10.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (9999999491) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 49996 + ... + 149998.

It is an arithmetic number, because the mean of its divisors is an integer number (882685306).

Almost surely, 2^9999999991 is an apocalyptic number.

9999999991 is a deficient number, since it is larger than the sum of its proper divisors (592223681).

9999999991 is a wasteful number, since it uses less digits than its factorization.

9999999991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 100318 (or 100299 counting only the distinct ones).

The product of its digits is 387420489, while the sum is 82.

The square root of 9999999991 is about 99999.9999550000. Note that the first 4 decimals coincide. The cubic root of 9999999991 is about 2154.4346893856.

It can be divided in two parts, 999999999 and 1, that added together give a 9-th power (1000000000 = 10⁹).

The spelling of 9999999991 in words is "nine billion, nine hundred ninety-nine million, nine hundred ninety-nine thousand, nine hundred ninety-one".