19981 has 8 divisors (see below), whose sum is σ = 22680. Its totient is φ = 17472.

The previous prime is 19979. The next prime is 19991. The reversal of 19981 is 18991.

It can be written as a sum of positive squares in 4 ways, for example, as 2025 + 17956 = 45^2 + 134^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 19981 - 2¹ = 19979 is a prime.

It is a super-2 number, since 2×19981² = 798480722, which contains 22 as substring.

It is an Ulam number.

It is a Duffinian number.

19981 is a lucky number.

It is a zygodrome in base 5.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (19961) 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 7 ways as a sum of consecutive naturals, for example, 351 + ... + 403.

It is an arithmetic number, because the mean of its divisors is an integer number (2835).

2¹⁹⁹⁸¹ is an apocalyptic number.

It is an amenable number.

19981 is a deficient number, since it is larger than the sum of its proper divisors (2699).

19981 is a wasteful number, since it uses less digits than its factorization.

19981 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 95.

The product of its digits is 648, while the sum is 28.

The square root of 19981 is about 141.3541651314. The cubic root of 19981 is about 27.1355777868.

Subtracting from 19981 its reverse (18991), we obtain a triangular number (990 = T₄₄).

It can be divided in two parts, 19 and 981, that added together give a cube (1000 = 10³).

The spelling of 19981 in words is "nineteen thousand, nine hundred eighty-one".