261556664756561 has 32 divisors (see below), whose sum is σ = 271443222640800. Its totient is φ = 251847800709120.

The previous prime is 261556664756537. The next prime is 261556664756581. The reversal of 261556664756561 is 165657466655162.

261556664756561 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It can be written as a sum of positive squares in 16 ways, for example, as 81407761890625 + 180148902865936 = 9022625^2 + 13421956^2 .

It is a cyclic number.

It is not a de Polignac number, because 261556664756561 - 2³⁴ = 261539484887377 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (261556664756581) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 14348373395 + ... + 14348391623.

It is an arithmetic number, because the mean of its divisors is an integer number (8482600707525).

Almost surely, 2^{261556664756561} is an apocalyptic number.

It is an amenable number.

261556664756561 is a deficient number, since it is larger than the sum of its proper divisors (9886557884239).

261556664756561 is a wasteful number, since it uses less digits than its factorization.

261556664756561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 25349.

The product of its digits is 1632960000, while the sum is 71.

The spelling of 261556664756561 in words is "two hundred sixty-one trillion, five hundred fifty-six billion, six hundred sixty-four million, seven hundred fifty-six thousand, five hundred sixty-one".