21601566 has 80 divisors (see below), whose sum is σ = 56732544. Its totient is φ = 6014736.

The previous prime is 21601561. The next prime is 21601579. The reversal of 21601566 is 66510612.

It is a happy number.

21601566 is a `hidden beast` number, since 2 + 1 + 601 + 56 + 6 = 666.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21601561) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 39 ways as a sum of consecutive naturals, for example, 48541 + ... + 48983.

Almost surely, 2^21601566 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21601566, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28366272).

21601566 is an abundant number, since it is smaller than the sum of its proper divisors (35130978).

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

21601566 is a wasteful number, since it uses less digits than its factorization.

21601566 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 507 (or 498 counting only the distinct ones).

The product of its (nonzero) digits is 2160, while the sum is 27.

The square root of 21601566 is about 4647.7484871710. The cubic root of 21601566 is about 278.5020601579.

The spelling of 21601566 in words is "twenty-one million, six hundred one thousand, five hundred sixty-six".