6639856121 has 2 divisors, whose sum is σ = 6639856122. Its totient is φ = 6639856120.

The previous prime is 6639856081. The next prime is 6639856163. The reversal of 6639856121 is 1216589366.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 6366284521 + 273571600 = 79789^2 + 16540^2 .

It is a cyclic number.

It is not a de Polignac number, because 6639856121 - 2¹⁰ = 6639855097 is a prime.

It is a super-4 number, since 4×6639856121⁴ (a number of 40 digits) contains 4444 as substring.

It is not a weakly prime, because it can be changed into another prime (6639856621) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 3319928060 + 3319928061.

It is an arithmetic number, because the mean of its divisors is an integer number (3319928061).

Almost surely, 2^6639856121 is an apocalyptic number.

It is an amenable number.

6639856121 is a deficient number, since it is larger than the sum of its proper divisors (1).

6639856121 is an equidigital number, since it uses as much as digits as its factorization.

6639856121 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 466560, while the sum is 47.

The square root of 6639856121 is about 81485.3123022794. The cubic root of 6639856121 is about 1879.5456991341.

The spelling of 6639856121 in words is "six billion, six hundred thirty-nine million, eight hundred fifty-six thousand, one hundred twenty-one".