256721 has 2 divisors, whose sum is σ = 256722. Its totient is φ = 256720.

The previous prime is 256699. The next prime is 256723. The reversal of 256721 is 127652.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 160000 + 96721 = 400^2 + 311^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-256721 is a prime.

Together with 256723, it forms a pair of twin primes.

It is a Chen prime.

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

It is a nialpdrome in base 8.

It is not a weakly prime, because it can be changed into another prime (256723) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 128360 + 128361.

It is an arithmetic number, because the mean of its divisors is an integer number (128361).

2²⁵⁶⁷²¹ is an apocalyptic number.

It is an amenable number.

256721 is a deficient number, since it is larger than the sum of its proper divisors (1).

256721 is an equidigital number, since it uses as much as digits as its factorization.

256721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 840, while the sum is 23.

The square root of 256721 is about 506.6764253446. The cubic root of 256721 is about 63.5555964205. Note that the first 4 decimals are identical.

The spelling of 256721 in words is "two hundred fifty-six thousand, seven hundred twenty-one".