256820821 has 2 divisors, whose sum is σ = 256820822. Its totient is φ = 256820820.

The previous prime is 256820813. The next prime is 256820857. The reversal of 256820821 is 128028652.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 246929796 + 9891025 = 15714^2 + 3145^2 .

It is a cyclic number.

It is not a de Polignac number, because 256820821 - 2³ = 256820813 is a prime.

It is a super-2 number, since 2×256820821² = 131913868198228082, which contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (256820021) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 128410410 + 128410411.

It is an arithmetic number, because the mean of its divisors is an integer number (128410411).

Almost surely, 2^256820821 is an apocalyptic number.

It is an amenable number.

256820821 is a deficient number, since it is larger than the sum of its proper divisors (1).

256820821 is an equidigital number, since it uses as much as digits as its factorization.

256820821 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 15360, while the sum is 34.

The square root of 256820821 is about 16025.6301280168. The cubic root of 256820821 is about 635.6383280827.

The spelling of 256820821 in words is "two hundred fifty-six million, eight hundred twenty thousand, eight hundred twenty-one".