21566640 has 80 divisors (see below), whose sum is σ = 69781248. Its totient is φ = 5499648.

The previous prime is 21566627. The next prime is 21566641. The reversal of 21566640 is 4666512.

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 = 21566598 and 21566607.

It is not an unprimeable number, because it can be changed into a prime (21566641) 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, 3567 + ... + 7473.

Almost surely, 2^21566640 is an apocalyptic number.

21566640 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 21566640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (34890624).

21566640 is an abundant number, since it is smaller than the sum of its proper divisors (48214608).

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

21566640 is a wasteful number, since it uses less digits than its factorization.

21566640 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3946 (or 3940 counting only the distinct ones).

The product of its (nonzero) digits is 8640, while the sum is 30.

The square root of 21566640 is about 4643.9896640712. The cubic root of 21566640 is about 278.3518826208.

The spelling of 21566640 in words is "twenty-one million, five hundred sixty-six thousand, six hundred forty".