251861561 has 4 divisors (see below), whose sum is σ = 287841792. Its totient is φ = 215881332.

The previous prime is 251861557. The next prime is 251861563. The reversal of 251861561 is 165168152.

It is a happy number.

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 251861561 - 2² = 251861557 is a prime.

It is not an unprimeable number, because it can be changed into a prime (251861563) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 17990105 + ... + 17990118.

It is an arithmetic number, because the mean of its divisors is an integer number (71960448).

Almost surely, 2^251861561 is an apocalyptic number.

It is an amenable number.

251861561 is a deficient number, since it is larger than the sum of its proper divisors (35980231).

251861561 is an equidigital number, since it uses as much as digits as its factorization.

251861561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 35980230.

The product of its digits is 14400, while the sum is 35.

The square root of 251861561 is about 15870.1468487220. The cubic root of 251861561 is about 631.5202731676.

The spelling of 251861561 in words is "two hundred fifty-one million, eight hundred sixty-one thousand, five hundred sixty-one".