21605640 has 192 divisors, whose sum is σ = 79574400. Its totient is φ = 4595712.

The previous prime is 21605627. The next prime is 21605659. The reversal of 21605640 is 4650612.

It is a Harshad number since it is a multiple of its sum of digits (24).

It is a self number, because there is not a number n which added to its sum of digits gives 21605640.

It is an unprimeable number.

It is a polite number, since it can be written in 47 ways as a sum of consecutive naturals, for example, 242716 + ... + 242804.

It is an arithmetic number, because the mean of its divisors is an integer number (414450).

Almost surely, 2^21605640 is an apocalyptic number.

21605640 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 21605640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (39787200).

21605640 is an abundant number, since it is smaller than the sum of its proper divisors (57968760).

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

21605640 is a wasteful number, since it uses less digits than its factorization.

21605640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 144 (or 123 counting only the distinct ones).

The product of its (nonzero) digits is 1440, while the sum is 24.

The square root of 21605640 is about 4648.1867432365. The cubic root of 21605640 is about 278.5195673157.

The spelling of 21605640 in words is "twenty-one million, six hundred five thousand, six hundred forty".