21555666 has 48 divisors (see below), whose sum is σ = 52668000. Its totient is φ = 6530040.

The previous prime is 21555647. The next prime is 21555673. The reversal of 21555666 is 66655512.

It is a happy number.

It is a self number, because there is not a number n which added to its sum of digits gives 21555666.

It is an unprimeable number.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 4885 + ... + 8183.

It is an arithmetic number, because the mean of its divisors is an integer number (1097250).

Almost surely, 2^21555666 is an apocalyptic number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21555666, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (26334000).

21555666 is an abundant number, since it is smaller than the sum of its proper divisors (31112334).

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

21555666 is a wasteful number, since it uses less digits than its factorization.

21555666 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3332 (or 3315 counting only the distinct ones).

The product of its digits is 54000, while the sum is 36.

The square root of 21555666 is about 4642.8079865530. The cubic root of 21555666 is about 278.3046622871.

The spelling of 21555666 in words is "twenty-one million, five hundred fifty-five thousand, six hundred sixty-six".