9891218721 has 16 divisors (see below), whose sum is σ = 14038401600. Its totient is φ = 6176890368.

The previous prime is 9891218707. The next prime is 9891218737. The reversal of 9891218721 is 1278121989.

9891218721 is digitally balanced in base 3, because in such base it contains all the possibile digits an equal number of times.

It is not a de Polignac number, because 9891218721 - 2⁶ = 9891218657 is a prime.

It is not an unprimeable number, because it can be changed into a prime (9891218021) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 969816 + ... + 979961.

It is an arithmetic number, because the mean of its divisors is an integer number (877400100).

Almost surely, 2^9891218721 is an apocalyptic number.

It is an amenable number.

9891218721 is a deficient number, since it is larger than the sum of its proper divisors (4147182879).

9891218721 is a wasteful number, since it uses less digits than its factorization.

9891218721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1949888.

The product of its digits is 145152, while the sum is 48.

The square root of 9891218721 is about 99454.6063337440. The cubic root of 9891218721 is about 2146.5941186464.

The spelling of 9891218721 in words is "nine billion, eight hundred ninety-one million, two hundred eighteen thousand, seven hundred twenty-one".