17991 has 6 divisors (see below), whose sum is σ = 26000. Its totient is φ = 11988.

The previous prime is 17989. The next prime is 18013. The reversal of 17991 is 19971.

It is not a de Polignac number, because 17991 - 2¹ = 17989 is a prime.

It is a Duffinian number.

17991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (17911) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 991 + ... + 1008.

2¹⁷⁹⁹¹ is an apocalyptic number.

17991 is a deficient number, since it is larger than the sum of its proper divisors (8009).

17991 is a wasteful number, since it uses less digits than its factorization.

17991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 2005 (or 2002 counting only the distinct ones).

The product of its digits is 567, while the sum is 27.

The square root of 17991 is about 134.1305334366. The cubic root of 17991 is about 26.2030453116.

Subtracting from 17991 its product of digits (567), we obtain a square (17424 = 132²).

The spelling of 17991 in words is "seventeen thousand, nine hundred ninety-one".