21005688 has 64 divisors (see below), whose sum is σ = 57697920. Its totient is φ = 6320000.

The previous prime is 21005681. The next prime is 21005689. The reversal of 21005688 is 88650012.

It is a Smith number, since the sum of its digits (30) coincides with the sum of the digits of its prime factors.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005681) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 66106 + ... + 66422.

It is an arithmetic number, because the mean of its divisors is an integer number (901530).

Almost surely, 2^21005688 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 21005688, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28848960).

21005688 is an abundant number, since it is smaller than the sum of its proper divisors (36692232).

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

21005688 is a wasteful number, since it uses less digits than its factorization.

21005688 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 588 (or 584 counting only the distinct ones).

The product of its (nonzero) digits is 3840, while the sum is 30.

The square root of 21005688 is about 4583.1962646171. The cubic root of 21005688 is about 275.9173245335.

The spelling of 21005688 in words is "twenty-one million, five thousand, six hundred eighty-eight".