251721 has 8 divisors (see below), whose sum is σ = 372960. Its totient is φ = 167796.

The previous prime is 251707. The next prime is 251737. The reversal of 251721 is 127152.

It is not a de Polignac number, because 251721 - 2¹⁴ = 235337 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 251694 and 251703.

It is not an unprimeable number, because it can be changed into a prime (251701) 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 in 7 ways as a sum of consecutive naturals, for example, 4635 + ... + 4688.

It is an arithmetic number, because the mean of its divisors is an integer number (46620).

2²⁵¹⁷²¹ is an apocalyptic number.

It is an amenable number.

251721 is a deficient number, since it is larger than the sum of its proper divisors (121239).

251721 is an equidigital number, since it uses as much as digits as its factorization.

251721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9332 (or 9326 counting only the distinct ones).

The product of its digits is 140, while the sum is 18.

The square root of 251721 is about 501.7180483100. The cubic root of 251721 is about 63.1402769939.

Adding to 251721 its reverse (127152), we get a palindrome (378873).

The spelling of 251721 in words is "two hundred fifty-one thousand, seven hundred twenty-one".