251891681 has 2 divisors, whose sum is σ = 251891682. Its totient is φ = 251891680.

The previous prime is 251891663. The next prime is 251891693. The reversal of 251891681 is 186198152.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 230584225 + 21307456 = 15185^2 + 4616^2 .

It is a cyclic number.

It is not a de Polignac number, because 251891681 - 2⁶ = 251891617 is a prime.

It is a super-2 number, since 2×251891681² = 126898837914011522, which contains 22 as substring.

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 (251891621) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 125945840 + 125945841.

It is an arithmetic number, because the mean of its divisors is an integer number (125945841).

Almost surely, 2^251891681 is an apocalyptic number.

It is an amenable number.

251891681 is a deficient number, since it is larger than the sum of its proper divisors (1).

251891681 is an equidigital number, since it uses as much as digits as its factorization.

251891681 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 34560, while the sum is 41.

The square root of 251891681 is about 15871.0957718741. The cubic root of 251891681 is about 631.5454465636.

The spelling of 251891681 in words is "two hundred fifty-one million, eight hundred ninety-one thousand, six hundred eighty-one".