21006656 has 56 divisors (see below), whose sum is σ = 46414944. Its totient is φ = 9351680.

The previous prime is 21006641. The next prime is 21006673. The reversal of 21006656 is 65660012.

It is a hoax number, since the sum of its digits (26) coincides with the sum of the digits of its distinct prime factors.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 37031 + ... + 37593.

Almost surely, 2^21006656 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 21006656, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (23207472).

21006656 is an abundant number, since it is smaller than the sum of its proper divisors (25408288).

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

21006656 is a wasteful number, since it uses less digits than its factorization.

21006656 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 639 (or 629 counting only the distinct ones).

The product of its (nonzero) digits is 2160, while the sum is 26.

The square root of 21006656 is about 4583.3018665586. The cubic root of 21006656 is about 275.9215628120.

Adding to 21006656 its reverse (65660012), we get a palindrome (86666668).

The spelling of 21006656 in words is "twenty-one million, six thousand, six hundred fifty-six".