256821 has 4 divisors (see below), whose sum is σ = 342432. Its totient is φ = 171212.

The previous prime is 256813. The next prime is 256831. The reversal of 256821 is 128652.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 256821 - 2³ = 256813 is a prime.

It is a D-number.

It is a junction number, because it is equal to n+sod(n) for n = 256791 and 256800.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (256801) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 42801 + ... + 42806.

It is an arithmetic number, because the mean of its divisors is an integer number (85608).

2²⁵⁶⁸²¹ is an apocalyptic number.

It is an amenable number.

256821 is a deficient number, since it is larger than the sum of its proper divisors (85611).

256821 is an equidigital number, since it uses as much as digits as its factorization.

256821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 85610.

The product of its digits is 960, while the sum is 24.

The square root of 256821 is about 506.7750980465. The cubic root of 256821 is about 63.5638475759.

The spelling of 256821 in words is "two hundred fifty-six thousand, eight hundred twenty-one".