21688784 has 40 divisors (see below), whose sum is σ = 45596040. Its totient is φ = 9993984.

The previous prime is 21688783. The next prime is 21688817. The reversal of 21688784 is 48788612.

It can be written as a sum of positive squares in 4 ways, for example, as 13162384 + 8526400 = 3628^2 + 2920^2 .

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21688783) 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, 34844 + ... + 35460.

It is an arithmetic number, because the mean of its divisors is an integer number (1139901).

Almost surely, 2^21688784 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 21688784, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (22798020).

21688784 is an abundant number, since it is smaller than the sum of its proper divisors (23907256).

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

21688784 is an equidigital number, since it uses as much as digits as its factorization.

21688784 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 664 (or 632 counting only the distinct ones).

The product of its digits is 172032, while the sum is 44.

The square root of 21688784 is about 4657.1218579719. The cubic root of 21688784 is about 278.8763813895.

The spelling of 21688784 in words is "twenty-one million, six hundred eighty-eight thousand, seven hundred eighty-four".