21644040 has 128 divisors (see below), whose sum is σ = 74649600. Its totient is φ = 4965120.

The previous prime is 21644039. The next prime is 21644041. The reversal of 21644040 is 4044612.

It is an interprime number because it is at equal distance from previous prime (21644039) and next prime (21644041).

It is a junction number, because it is equal to n+sod(n) for n = 21643998 and 21644016.

It is not an unprimeable number, because it can be changed into a prime (21644041) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 24649 + ... + 25511.

It is an arithmetic number, because the mean of its divisors is an integer number (583200).

Almost surely, 2^21644040 is an apocalyptic number.

21644040 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 21644040, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (37324800).

21644040 is an abundant number, since it is smaller than the sum of its proper divisors (53005560).

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

21644040 is a wasteful number, since it uses less digits than its factorization.

21644040 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 907 (or 903 counting only the distinct ones).

The product of its (nonzero) digits is 768, while the sum is 21.

The square root of 21644040 is about 4652.3155524964. The cubic root of 21644040 is about 278.6844752043.

Adding to 21644040 its reverse (4044612), we get a palindrome (25688652).

The spelling of 21644040 in words is "twenty-one million, six hundred forty-four thousand, forty".