11981 has 2 divisors, whose sum is σ = 11982. Its totient is φ = 11980.

The previous prime is 11971. The next prime is 11987. The reversal of 11981 is 18911.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 11881 + 100 = 109^2 + 10^2 .

It is an emirp because it is prime and its reverse (18911) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 11981 - 2¹⁰ = 10957 is a prime.

It is a super-2 number, since 2×11981² = 287088722, which contains 22 as substring.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (11987) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5990 + 5991.

It is an arithmetic number, because the mean of its divisors is an integer number (5991).

2¹¹⁹⁸¹ is an apocalyptic number.

It is an amenable number.

11981 is a deficient number, since it is larger than the sum of its proper divisors (1).

11981 is an equidigital number, since it uses as much as digits as its factorization.

11981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 72, while the sum is 20.

The square root of 11981 is about 109.4577544078. The cubic root of 11981 is about 22.8821953735.

The spelling of 11981 in words is "eleven thousand, nine hundred eighty-one".