81981 has 6 divisors (see below), whose sum is σ = 118430. Its totient is φ = 54648.

The previous prime is 81973. The next prime is 82003. The reversal of 81981 is 18918.

It can be written as a sum of positive squares in only one way, i.e., 54756 + 27225 = 234^2 + 165^2 .

It is not a de Polignac number, because 81981 - 2³ = 81973 is a prime.

It is a super-2 number, since 2×81981² = 13441768722, which contains 22 as substring.

It is an Ulam number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81901) 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, 4546 + ... + 4563.

2⁸¹⁹⁸¹ is an apocalyptic number.

It is an amenable number.

81981 is a deficient number, since it is larger than the sum of its proper divisors (36449).

81981 is a wasteful number, since it uses less digits than its factorization.

81981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9115 (or 9112 counting only the distinct ones).

The product of its digits is 576, while the sum is 27.

The square root of 81981 is about 286.3232439045. The cubic root of 81981 is about 43.4414591047.

It can be divided in two parts, 819 and 81, that added together give a square (900 = 30²).

The spelling of 81981 in words is "eighty-one thousand, nine hundred eighty-one".