21206656656 has 320 divisors, whose sum is σ = 65595801600. Its totient is φ = 6621886080.

The previous prime is 21206656631. The next prime is 21206656657. The reversal of 21206656656 is 65665660212.

It is a happy number.

21206656656 is a `hidden beast` number, since 2 + 1 + 20 + 66 + 566 + 5 + 6 = 666.

It is not an unprimeable number, because it can be changed into a prime (21206656657) by changing a digit.

It is a polite number, since it can be written in 63 ways as a sum of consecutive naturals, for example, 111029521 + ... + 111029711.

It is an arithmetic number, because the mean of its divisors is an integer number (204986880).

Almost surely, 2^21206656656 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21206656656, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32797900800).

21206656656 is an abundant number, since it is smaller than the sum of its proper divisors (44389144944).

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

21206656656 is a wasteful number, since it uses less digits than its factorization.

21206656656 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 406 (or 382 counting only the distinct ones).

The product of its (nonzero) digits is 777600, while the sum is 45.

The spelling of 21206656656 in words is "twenty-one billion, two hundred six million, six hundred fifty-six thousand, six hundred fifty-six".