21216636 has 72 divisors (see below), whose sum is σ = 62375040. Its totient is φ = 5954976.

The previous prime is 21216631. The next prime is 21216659. The reversal of 21216636 is 63661212.

21216636 is a `hidden beast` number, since 2 + 1 + 21 + 6 + 636 = 666.

It is a nude number because it is divisible by every one of its digits.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21216631) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 14155 + ... + 15581.

It is an arithmetic number, because the mean of its divisors is an integer number (866320).

Almost surely, 2^21216636 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 21216636, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (31187520).

21216636 is an abundant number, since it is smaller than the sum of its proper divisors (41158404).

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

21216636 is a wasteful number, since it uses less digits than its factorization.

21216636 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1503 (or 1498 counting only the distinct ones).

The product of its digits is 2592, while the sum is 27.

The square root of 21216636 is about 4606.1519731767. The cubic root of 21216636 is about 276.8378759969.

Adding to 21216636 its reverse (63661212), we get a palindrome (84877848).

The spelling of 21216636 in words is "twenty-one million, two hundred sixteen thousand, six hundred thirty-six".