256644673808561 has 2 divisors, whose sum is σ = 256644673808562. Its totient is φ = 256644673808560.

The previous prime is 256644673808543. The next prime is 256644673808591. The reversal of 256644673808561 is 165808376446652.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 143845001731600 + 112799672076961 = 11993540^2 + 10620719^2 .

It is a cyclic number.

It is not a de Polignac number, because 256644673808561 - 2²⁶ = 256644606699697 is a prime.

It is not a weakly prime, because it can be changed into another prime (256644673808591) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 128322336904280 + 128322336904281.

It is an arithmetic number, because the mean of its divisors is an integer number (128322336904281).

Almost surely, 2^{256644673808561} is an apocalyptic number.

It is an amenable number.

256644673808561 is a deficient number, since it is larger than the sum of its proper divisors (1).

256644673808561 is an equidigital number, since it uses as much as digits as its factorization.

256644673808561 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 1393459200, while the sum is 71.

The spelling of 256644673808561 in words is "two hundred fifty-six trillion, six hundred forty-four billion, six hundred seventy-three million, eight hundred eight thousand, five hundred sixty-one".