251661 has 8 divisors (see below), whose sum is σ = 338400. Its totient is φ = 166352.

The previous prime is 251653. The next prime is 251663. The reversal of 251661 is 166152.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 251661 - 2³ = 251653 is a prime.

It is an Ulam number.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 251661.

It is not an unprimeable number, because it can be changed into a prime (251663) 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, 166 + ... + 728.

It is an arithmetic number, because the mean of its divisors is an integer number (42300).

2²⁵¹⁶⁶¹ is an apocalyptic number.

It is an amenable number.

251661 is a deficient number, since it is larger than the sum of its proper divisors (86739).

251661 is a wasteful number, since it uses less digits than its factorization.

251661 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 715.

The product of its digits is 360, while the sum is 21.

The square root of 251661 is about 501.6582502063. The cubic root of 251661 is about 63.1352599080.

The spelling of 251661 in words is "two hundred fifty-one thousand, six hundred sixty-one".