256561 has 2 divisors, whose sum is σ = 256562. Its totient is φ = 256560.

The previous prime is 256541. The next prime is 256567. The reversal of 256561 is 165652.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 250000 + 6561 = 500^2 + 81^2 .

It is a cyclic number.

It is not a de Polignac number, because 256561 - 2⁹ = 256049 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is not a weakly prime, because it can be changed into another prime (256567) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 128280 + 128281.

It is an arithmetic number, because the mean of its divisors is an integer number (128281).

2²⁵⁶⁵⁶¹ is an apocalyptic number.

It is an amenable number.

256561 is a deficient number, since it is larger than the sum of its proper divisors (1).

256561 is an equidigital number, since it uses as much as digits as its factorization.

256561 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1800, while the sum is 25.

The square root of 256561 is about 506.5185090399. The cubic root of 256561 is about 63.5423901139.

Subtracting from 256561 its reverse (165652), we obtain a palindrome (90909).

It can be divided in two parts, 25 and 6561, that multiplied together give a square (164025 = 405²).

The spelling of 256561 in words is "two hundred fifty-six thousand, five hundred sixty-one".