251561 has 2 divisors, whose sum is σ = 251562. Its totient is φ = 251560.

The previous prime is 251543. The next prime is 251567. The reversal of 251561 is 165152.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 133225 + 118336 = 365^2 + 344^2 .

It is a cyclic number.

It is not a de Polignac number, because 251561 - 2¹⁴ = 235177 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is not a weakly prime, because it can be changed into another prime (251567) 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, 125780 + 125781.

It is an arithmetic number, because the mean of its divisors is an integer number (125781).

2²⁵¹⁵⁶¹ is an apocalyptic number.

It is an amenable number.

251561 is a deficient number, since it is larger than the sum of its proper divisors (1).

251561 is an equidigital number, since it uses as much as digits as its factorization.

251561 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 300, while the sum is 20.

The square root of 251561 is about 501.5585708569. The cubic root of 251561 is about 63.1268963257.

The spelling of 251561 in words is "two hundred fifty-one thousand, five hundred sixty-one".