20013721565561 has 2 divisors, whose sum is σ = 20013721565562. Its totient is φ = 20013721565560.

The previous prime is 20013721565557. The next prime is 20013721565611. The reversal of 20013721565561 is 16556512731002.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18059261645161 + 1954459920400 = 4249619^2 + 1398020^2 .

It is a cyclic number.

It is not a de Polignac number, because 20013721565561 - 2² = 20013721565557 is a prime.

It is not a weakly prime, because it can be changed into another prime (20013721564561) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10006860782780 + 10006860782781.

It is an arithmetic number, because the mean of its divisors is an integer number (10006860782781).

Almost surely, 2^{20013721565561} is an apocalyptic number.

It is an amenable number.

20013721565561 is a deficient number, since it is larger than the sum of its proper divisors (1).

20013721565561 is an equidigital number, since it uses as much as digits as its factorization.

20013721565561 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 378000, while the sum is 44.

The spelling of 20013721565561 in words is "twenty trillion, thirteen billion, seven hundred twenty-one million, five hundred sixty-five thousand, five hundred sixty-one".