21688566 has 16 divisors (see below), whose sum is σ = 45928944. Its totient is φ = 6804224.

The previous prime is 21688559. The next prime is 21688567. The reversal of 21688566 is 66588612.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21688567) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 106215 + ... + 106418.

It is an arithmetic number, because the mean of its divisors is an integer number (2870559).

Almost surely, 2^21688566 is an apocalyptic number.

21688566 is an abundant number, since it is smaller than the sum of its proper divisors (24240378).

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

21688566 is a wasteful number, since it uses less digits than its factorization.

21688566 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 212655.

The product of its digits is 138240, while the sum is 42.

The square root of 21688566 is about 4657.0984528996. The cubic root of 21688566 is about 278.8754470317.

The spelling of 21688566 in words is "twenty-one million, six hundred eighty-eight thousand, five hundred sixty-six".