3128721921 has 24 divisors (see below), whose sum is σ = 4870558056. Its totient is φ = 1923920640.

The previous prime is 3128721881. The next prime is 3128721941. The reversal of 3128721921 is 1291278213.

It can be written as a sum of positive squares in 4 ways, for example, as 56791296 + 3071930625 = 7536^2 + 55425^2 .

It is not a de Polignac number, because 3128721921 - 2⁷ = 3128721793 is a prime.

It is not an unprimeable number, because it can be changed into a prime (3128721941) 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 23 ways as a sum of consecutive naturals, for example, 158331 + ... + 176991.

It is an arithmetic number, because the mean of its divisors is an integer number (202939919).

Almost surely, 2^3128721921 is an apocalyptic number.

It is an amenable number.

3128721921 is a deficient number, since it is larger than the sum of its proper divisors (1741836135).

3128721921 is a wasteful number, since it uses less digits than its factorization.

3128721921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20113 (or 20110 counting only the distinct ones).

The product of its digits is 12096, while the sum is 36.

The square root of 3128721921 is about 55934.9794046623. The cubic root of 3128721921 is about 1462.5890635220.

The spelling of 3128721921 in words is "three billion, one hundred twenty-eight million, seven hundred twenty-one thousand, nine hundred twenty-one".