9991821 has 32 divisors (see below), whose sum is σ = 16109568. Its totient is φ = 5385600.

The previous prime is 9991811. The next prime is 9991837. The reversal of 9991821 is 1281999.

It is a happy number.

9991821 is digitally balanced in base 2, base 3 and base 8, because in such bases it contains all the possibile digits an equal number of times.

It is not a de Polignac number, because 9991821 - 2⁶ = 9991757 is a prime.

9991821 is strictly pandigital in base 8.

It is a self number, because there is not a number n which added to its sum of digits gives 9991821.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (9991811) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 66096 + ... + 66246.

It is an arithmetic number, because the mean of its divisors is an integer number (503424).

Almost surely, 2^9991821 is an apocalyptic number.

It is an amenable number.

9991821 is a deficient number, since it is larger than the sum of its proper divisors (6117747).

9991821 is a wasteful number, since it uses less digits than its factorization.

9991821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 321.

The product of its digits is 11664, while the sum is 39.

The square root of 9991821 is about 3160.9841821812. The cubic root of 9991821 is about 215.3847159111.

The spelling of 9991821 in words is "nine million, nine hundred ninety-one thousand, eight hundred twenty-one".