9991921 has 9 divisors (see below), whose sum is σ = 10444161. Its totient is φ = 9558864.

The previous prime is 9991879. The next prime is 9991939. The reversal of 9991921 is 1291999.

The square root of 9991921 is 3161.

It is a perfect power (a square), and thus also a powerful number.

It can be written as a sum of positive squares in 4 ways, for example, as 505521 + 9486400 = 711^2 + 3080^2 .

It is not a de Polignac number, because 9991921 - 2⁹ = 9991409 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (9992921) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 91615 + ... + 91723.

Almost surely, 2^9991921 is an apocalyptic number.

9991921 is the 3161-st square number.

9991921 is the 1581-st centered octagonal number.

It is an amenable number.

9991921 is a deficient number, since it is larger than the sum of its proper divisors (452240).

9991921 is an equidigital number, since it uses as much as digits as its factorization.

9991921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 276 (or 138 counting only the distinct ones).

The product of its digits is 13122, while the sum is 40.

The cubic root of 9991921 is about 215.3854344455.

Subtracting from 9991921 its product of digits (13122), we obtain a palindrome (9978799).

The spelling of 9991921 in words is "nine million, nine hundred ninety-one thousand, nine hundred twenty-one".