17981 has 2 divisors, whose sum is σ = 17982. Its totient is φ = 17980.

The previous prime is 17977. The next prime is 17987. The reversal of 17981 is 18971.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 17956 + 25 = 134^2 + 5^2 .

It is a cyclic number.

It is not a de Polignac number, because 17981 - 2² = 17977 is a prime.

It is a super-2 number, since 2×17981² = 646632722, which contains 22 as substring.

It is a Sophie Germain prime.

It is a Curzon number.

It is a plaindrome in base 11.

It is a self number, because there is not a number n which added to its sum of digits gives 17981.

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 17981.

It is not a weakly prime, because it can be changed into another prime (17987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 8990 + 8991.

It is an arithmetic number, because the mean of its divisors is an integer number (8991).

2¹⁷⁹⁸¹ is an apocalyptic number.

It is an amenable number.

17981 is a deficient number, since it is larger than the sum of its proper divisors (1).

17981 is an equidigital number, since it uses as much as digits as its factorization.

17981 is an evil number, because the sum of its binary digits is even.

The product of its digits is 504, while the sum is 26.

The square root of 17981 is about 134.0932511352. The cubic root of 17981 is about 26.1981895686.

Subtracting from 17981 its sum of digits (26), we obtain a triangular number (17955 = T₁₈₉).

Subtracting 17981 from its reverse (18971), we obtain a triangular number (990 = T₄₄).

The spelling of 17981 in words is "seventeen thousand, nine hundred eighty-one".