9891991987 has 2 divisors, whose sum is σ = 9891991988. Its totient is φ = 9891991986.

The previous prime is 9891991973. The next prime is 9891991999. The reversal of 9891991987 is 7891991989.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 9891991987 - 2²⁷ = 9757774259 is a prime.

It is not a weakly prime, because it can be changed into another prime (9891991387) 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 as a sum of consecutive naturals, namely, 4945995993 + 4945995994.

It is an arithmetic number, because the mean of its divisors is an integer number (4945995994).

Almost surely, 2^9891991987 is an apocalyptic number.

9891991987 is a deficient number, since it is larger than the sum of its proper divisors (1).

9891991987 is an equidigital number, since it uses as much as digits as its factorization.

9891991987 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 26453952, while the sum is 70.

The square root of 9891991987 is about 99458.4937901233. The cubic root of 9891991987 is about 2146.6500552990.

The spelling of 9891991987 in words is "nine billion, eight hundred ninety-one million, nine hundred ninety-one thousand, nine hundred eighty-seven".