256991 has 4 divisors (see below), whose sum is σ = 293712. Its totient is φ = 220272.

The previous prime is 256981. The next prime is 257003. The reversal of 256991 is 199652.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 256991 - 2¹⁴ = 240607 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (256901) 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, 18350 + ... + 18363.

It is an arithmetic number, because the mean of its divisors is an integer number (73428).

2²⁵⁶⁹⁹¹ is an apocalyptic number.

256991 is a deficient number, since it is larger than the sum of its proper divisors (36721).

256991 is an equidigital number, since it uses as much as digits as its factorization.

256991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 36720.

The product of its digits is 4860, while the sum is 32.

The square root of 256991 is about 506.9427975620. The cubic root of 256991 is about 63.5778696259.

The spelling of 256991 in words is "two hundred fifty-six thousand, nine hundred ninety-one".