165991 has 8 divisors (see below), whose sum is σ = 198144. Its totient is φ = 135960.

The previous prime is 165983. The next prime is 166013. The reversal of 165991 is 199561.

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 165991 - 2³ = 165983 is a prime.

It is a Duffinian number.

165991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (165901) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 355 + ... + 676.

It is an arithmetic number, because the mean of its divisors is an integer number (24768).

2¹⁶⁵⁹⁹¹ is an apocalyptic number.

165991 is a deficient number, since it is larger than the sum of its proper divisors (32153).

165991 is a wasteful number, since it uses less digits than its factorization.

165991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1061.

The product of its digits is 2430, while the sum is 31.

The square root of 165991 is about 407.4199307840. The cubic root of 165991 is about 54.9576533544.

It can be divided in two parts, 165 and 991, that added together give a square (1156 = 34²).

The spelling of 165991 in words is "one hundred sixty-five thousand, nine hundred ninety-one".