9891991988 has 12 divisors (see below), whose sum is σ = 17907916740. Its totient is φ = 4775444352.

The previous prime is 9891991987. The next prime is 9891991999. The reversal of 9891991988 is 8891991989.

It can be written as a sum of positive squares in 2 ways, for example, as 7960565284 + 1931426704 = 89222^2 + 43948^2 .

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (9891991987) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 42637781 + ... + 42638012.

It is an arithmetic number, because the mean of its divisors is an integer number (1492326395).

Almost surely, 2^9891991988 is an apocalyptic number.

It is an amenable number.

9891991988 is a deficient number, since it is larger than the sum of its proper divisors (8015924752).

9891991988 is a wasteful number, since it uses less digits than its factorization.

9891991988 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 85275826 (or 85275824 counting only the distinct ones).

The product of its digits is 30233088, while the sum is 71.

The square root of 9891991988 is about 99458.4937951505. The cubic root of 9891991988 is about 2146.6500553713.

Subtracting from 9891991988 its reverse (8891991989), we obtain a palindrome (999999999).

The spelling of 9891991988 in words is "nine billion, eight hundred ninety-one million, nine hundred ninety-one thousand, nine hundred eighty-eight".