251991 has 24 divisors (see below), whose sum is σ = 406224. Its totient is φ = 155520.

The previous prime is 251983. The next prime is 252001. The reversal of 251991 is 199152.

It is a happy number.

It is not a de Polignac number, because 251991 - 2³ = 251983 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

Its product of digits (810) is a multiple of the sum of its prime divisors (81).

251991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (251941) 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 23 ways as a sum of consecutive naturals, for example, 4101 + ... + 4161.

It is an arithmetic number, because the mean of its divisors is an integer number (16926).

2²⁵¹⁹⁹¹ is an apocalyptic number.

251991 is a deficient number, since it is larger than the sum of its proper divisors (154233).

251991 is an equidigital number, since it uses as much as digits as its factorization.

251991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 93 (or 81 counting only the distinct ones).

The product of its digits is 810, while the sum is 27.

The square root of 251991 is about 501.9870516258. The cubic root of 251991 is about 63.1628440200.

It can be divided in two parts, 25199 and 1, that added together give a triangular number (25200 = T₂₂₄).

The spelling of 251991 in words is "two hundred fifty-one thousand, nine hundred ninety-one".