19821 has 4 divisors (see below), whose sum is σ = 26432. Its totient is φ = 13212.

The previous prime is 19819. The next prime is 19841. The reversal of 19821 is 12891.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4, and also an emirpimes, since its reverse is a distinct semiprime: 12891 = 3 ⋅4297.

It is not a de Polignac number, because 19821 - 2¹ = 19819 is a prime.

It is a D-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 (19801) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3301 + ... + 3306.

It is an arithmetic number, because the mean of its divisors is an integer number (6608).

2¹⁹⁸²¹ is an apocalyptic number.

It is an amenable number.

19821 is a deficient number, since it is larger than the sum of its proper divisors (6611).

19821 is an equidigital number, since it uses as much as digits as its factorization.

19821 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6610.

The product of its digits is 144, while the sum is 21.

The square root of 19821 is about 140.7870732702. The cubic root of 19821 is about 27.0629532412.

The spelling of 19821 in words is "nineteen thousand, eight hundred twenty-one".