21204 has 36 divisors (see below), whose sum is σ = 58240. Its totient is φ = 6480.

The previous prime is 21193. The next prime is 21211. The reversal of 21204 is 40212.

It is a tau number, because it is divible by the number of its divisors (36).

It is a Harshad number since it is a multiple of its sum of digits (9).

It is a nialpdrome in base 13.

It is a congruent number.

It is an unprimeable number.

21204 is an untouchable number, because it is not equal to the sum of proper divisors of any number.

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 11 ways as a sum of consecutive naturals, for example, 669 + ... + 699.

2²¹²⁰⁴ is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21204, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (29120).

21204 is an abundant number, since it is smaller than the sum of its proper divisors (37036).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

21204 is a wasteful number, since it uses less digits than its factorization.

21204 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 60 (or 55 counting only the distinct ones).

The product of its (nonzero) digits is 16, while the sum is 9.

The square root of 21204 is about 145.6159331941. The cubic root of 21204 is about 27.6782906270.

Adding to 21204 its reverse (40212), we get a palindrome (61416).

It can be divided in two parts, 21 and 204, that added together give a square (225 = 15²).

The spelling of 21204 in words is "twenty-one thousand, two hundred four", and thus it is an iban number.