3991721820 has 192 divisors, whose sum is σ = 12219863040. Its totient is φ = 970905600.

The previous prime is 3991721807. The next prime is 3991721827. The reversal of 3991721820 is 281271993.

It is a super-2 number, since 2×3991721820² = 31867686176528224800, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (3991721827) by changing a digit.

It is a polite number, since it can be written in 63 ways as a sum of consecutive naturals, for example, 2318560 + ... + 2320280.

It is an arithmetic number, because the mean of its divisors is an integer number (63645120).

Almost surely, 2^3991721820 is an apocalyptic number.

3991721820 is a gapful number since it is divisible by the number (30) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 3991721820, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (6109931520).

3991721820 is an abundant number, since it is smaller than the sum of its proper divisors (8228141220).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

3991721820 is a wasteful number, since it uses less digits than its factorization.

3991721820 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1836 (or 1834 counting only the distinct ones).

The product of its (nonzero) digits is 54432, while the sum is 42.

The square root of 3991721820 is about 63180.0745488639. The cubic root of 3991721820 is about 1586.3052296990.

The spelling of 3991721820 in words is "three billion, nine hundred ninety-one million, seven hundred twenty-one thousand, eight hundred twenty".