21664566 has 144 divisors (see below), whose sum is σ = 62244000. Its totient is φ = 5433120.

The previous prime is 21664529. The next prime is 21664607. The reversal of 21664566 is 66546612.

It is a happy number.

21664566 is a `hidden beast` number, since 2 + 1 + 6 + 645 + 6 + 6 = 666.

Its product of digits (51840) is a multiple of the sum of its prime factors (80).

It is a self number, because there is not a number n which added to its sum of digits gives 21664566.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 71 ways as a sum of consecutive naturals, for example, 747040 + ... + 747068.

It is an arithmetic number, because the mean of its divisors is an integer number (432250).

Almost surely, 2^21664566 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21664566, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (31122000).

21664566 is an abundant number, since it is smaller than the sum of its proper divisors (40579434).

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

21664566 is a wasteful number, since it uses less digits than its factorization.

21664566 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 80 (or 52 counting only the distinct ones).

The product of its digits is 51840, while the sum is 36.

The square root of 21664566 is about 4654.5210279899. The cubic root of 21664566 is about 278.7725436398.

The spelling of 21664566 in words is "twenty-one million, six hundred sixty-four thousand, five hundred sixty-six".