21657636 has 144 divisors (see below), whose sum is σ = 73627008. Its totient is φ = 5184000.

The previous prime is 21657631. The next prime is 21657637. The reversal of 21657636 is 63675612.

21657636 is a `hidden beast` number, since 2 + 16 + 5 + 7 + 636 = 666.

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a self number, because there is not a number n which added to its sum of digits gives 21657636.

It is not an unprimeable number, because it can be changed into a prime (21657631) by changing a digit.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 35736 + ... + 36336.

Almost surely, 2^21657636 is an apocalyptic number.

21657636 is a gapful number since it is divisible by the number (26) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21657636, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (36813504).

21657636 is an abundant number, since it is smaller than the sum of its proper divisors (51969372).

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

21657636 is a wasteful number, since it uses less digits than its factorization.

21657636 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 642 (or 637 counting only the distinct ones).

The product of its digits is 45360, while the sum is 36.

The square root of 21657636 is about 4653.7765309477. The cubic root of 21657636 is about 278.7428161460.

It can be divided in two parts, 216576 and 36, that added together give a palindrome (216612).

The spelling of 21657636 in words is "twenty-one million, six hundred fifty-seven thousand, six hundred thirty-six".