21891636 has 72 divisors (see below), whose sum is σ = 61687080. Its totient is φ = 6499584.

The previous prime is 21891599. The next prime is 21891637. The reversal of 21891636 is 63619812.

21891636 is a `hidden beast` number, since 2 + 18 + 9 + 1 + 636 = 666.

It can be written as a sum of positive squares in 4 ways, for example, as 1587600 + 20304036 = 1260^2 + 4506^2 .

It is a Smith number, since the sum of its digits (36) coincides with the sum of the digits of its prime factors.

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 21891636.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21891637) 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, 12766 + ... + 14378.

It is an arithmetic number, because the mean of its divisors is an integer number (856765).

Almost surely, 2^21891636 is an apocalyptic number.

21891636 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 21891636, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (30843540).

21891636 is an abundant number, since it is smaller than the sum of its proper divisors (39795444).

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

21891636 is a wasteful number, since it uses less digits than its factorization.

21891636 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1665 (or 1660 counting only the distinct ones).

The product of its digits is 15552, while the sum is 36.

The square root of 21891636 is about 4678.8498586725. The cubic root of 21891636 is about 279.7431147335.

The spelling of 21891636 in words is "twenty-one million, eight hundred ninety-one thousand, six hundred thirty-six".