21638640 has 80 divisors (see below), whose sum is σ = 69415200. Its totient is φ = 5569536.

The previous prime is 21638633. The next prime is 21638647. The reversal of 21638640 is 4683612.

It is an interprime number because it is at equal distance from previous prime (21638633) and next prime (21638647).

It is a tau number, because it is divible by the number of its divisors (80).

It is a Harshad number since it is a multiple of its sum of digits (30).

It is a junction number, because it is equal to n+sod(n) for n = 21638598 and 21638607.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21638647) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 5406 + ... + 8514.

It is an arithmetic number, because the mean of its divisors is an integer number (867690).

Almost surely, 2^21638640 is an apocalyptic number.

21638640 is a gapful number since it is divisible by the number (20) 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 21638640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (34707600).

21638640 is an abundant number, since it is smaller than the sum of its proper divisors (47776560).

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

21638640 is a wasteful number, since it uses less digits than its factorization.

21638640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3154 (or 3148 counting only the distinct ones).

The product of its (nonzero) digits is 6912, while the sum is 30.

The square root of 21638640 is about 4651.7351601311. The cubic root of 21638640 is about 278.6612968245.

The spelling of 21638640 in words is "twenty-one million, six hundred thirty-eight thousand, six hundred forty".