21204656 has 40 divisors (see below), whose sum is σ = 45110208. Its totient is φ = 9576000.

The previous prime is 21204641. The next prime is 21204679. The reversal of 21204656 is 65640212.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 36851 + ... + 37421.

Almost surely, 2^21204656 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 21204656, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (22555104).

21204656 is an abundant number, since it is smaller than the sum of its proper divisors (23905552).

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

21204656 is a wasteful number, since it uses less digits than its factorization.

21204656 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 801 (or 795 counting only the distinct ones).

The product of its (nonzero) digits is 2880, while the sum is 26.

The square root of 21204656 is about 4604.8513548213. The cubic root of 21204656 is about 276.7857605692.

Adding to 21204656 its reverse (65640212), we get a palindrome (86844868).

Subtracting 21204656 from its reverse (65640212), we obtain a square (44435556 = 6666²).

It can be divided in two parts, 2120 and 4656, that added together give a palindrome (6776).

The spelling of 21204656 in words is "twenty-one million, two hundred four thousand, six hundred fifty-six".