251691852721 has 9 divisors (see below), whose sum is σ = 252564288249. Its totient is φ = 250822425684.

The previous prime is 251691852707. The next prime is 251691852733. The reversal of 251691852721 is 127258196152.

The square root of 251691852721 is 501689.

It is a perfect power (a square), and thus also a powerful number.

It is not a de Polignac number, because 251691852721 - 2¹⁹ = 251691328433 is a prime.

It is a super-2 number, since 2×251691852721² (a number of 24 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (251691852121) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 184119180 + ... + 184120546.

Almost surely, 2^251691852721 is an apocalyptic number.

251691852721 is the 501689-th square number.

251691852721 is the 250845-th centered octagonal number.

It is an amenable number.

251691852721 is a deficient number, since it is larger than the sum of its proper divisors (872435528).

251691852721 is an frugal number, since it uses more digits than its factorization.

251691852721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3468 (or 1734 counting only the distinct ones).

The product of its digits is 604800, while the sum is 49.

Multiplying 251691852721 by its sum of digits (49), we get a square (12332900783329 = 3511823²).

The spelling of 251691852721 in words is "two hundred fifty-one billion, six hundred ninety-one million, eight hundred fifty-two thousand, seven hundred twenty-one".