20639646721 has 4 divisors (see below), whose sum is σ = 20645769172. Its totient is φ = 20633524272.

The previous prime is 20639646707. The next prime is 20639646757. The reversal of 20639646721 is 12764693602.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 19813659121 + 825987600 = 140761^2 + 28740^2 .

It is a cyclic number.

It is not a de Polignac number, because 20639646721 - 2⁷ = 20639646593 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20639649721) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3056166 + ... + 3062911.

It is an arithmetic number, because the mean of its divisors is an integer number (5161442293).

Almost surely, 2^20639646721 is an apocalyptic number.

It is an amenable number.

20639646721 is a deficient number, since it is larger than the sum of its proper divisors (6122451).

20639646721 is an equidigital number, since it uses as much as digits as its factorization.

20639646721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6122450.

The product of its (nonzero) digits is 653184, while the sum is 46.

The spelling of 20639646721 in words is "twenty billion, six hundred thirty-nine million, six hundred forty-six thousand, seven hundred twenty-one".