6561521557 has 2 divisors, whose sum is σ = 6561521558. Its totient is φ = 6561521556.

The previous prime is 6561521537. The next prime is 6561521563. The reversal of 6561521557 is 7551251656.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 6210858481 + 350663076 = 78809^2 + 18726^2 .

It is a cyclic number.

It is not a de Polignac number, because 6561521557 - 2²⁷ = 6427303829 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (6561521507) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 3280760778 + 3280760779.

It is an arithmetic number, because the mean of its divisors is an integer number (3280760779).

Almost surely, 2^6561521557 is an apocalyptic number.

It is an amenable number.

6561521557 is a deficient number, since it is larger than the sum of its proper divisors (1).

6561521557 is an equidigital number, since it uses as much as digits as its factorization.

6561521557 is an evil number, because the sum of its binary digits is even.

The product of its digits is 315000, while the sum is 43.

The square root of 6561521557 is about 81003.2194236748. The cubic root of 6561521557 is about 1872.1250453893.

The spelling of 6561521557 in words is "six billion, five hundred sixty-one million, five hundred twenty-one thousand, five hundred fifty-seven".