20981 has 2 divisors, whose sum is σ = 20982. Its totient is φ = 20980.

The previous prime is 20963. The next prime is 20983. The reversal of 20981 is 18902.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17956 + 3025 = 134^2 + 55^2 .

It is a cyclic number.

It is not a de Polignac number, because 20981 - 2¹⁴ = 4597 is a prime.

It is a super-2 number, since 2×20981² = 880404722, which contains 22 as substring.

Together with 20983, it forms a pair of twin primes.

It is a Chen prime.

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 20981.

It is not a weakly prime, because it can be changed into another prime (20983) by changing a digit.

It is a good prime.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10490 + 10491.

It is an arithmetic number, because the mean of its divisors is an integer number (10491).

2²⁰⁹⁸¹ is an apocalyptic number.

It is an amenable number.

20981 is a deficient number, since it is larger than the sum of its proper divisors (1).

20981 is an equidigital number, since it uses as much as digits as its factorization.

20981 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 144, while the sum is 20.

The square root of 20981 is about 144.8481963989. The cubic root of 20981 is about 27.5809186882.

It can be divided in two parts, 20 and 981, that added together give a palindrome (1001).

The spelling of 20981 in words is "twenty thousand, nine hundred eighty-one".