21817728 has 128 divisors (see below), whose sum is σ = 66096000. Its totient is φ = 7082496.

The previous prime is 21817723. The next prime is 21817759. The reversal of 21817728 is 82771812.

It is a happy number.

It is a tau number, because it is divible by the number of its divisors (128).

It is a Harshad number since it is a multiple of its sum of digits (36).

It is a junction number, because it is equal to n+sod(n) for n = 21817692 and 21817701.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21817721) 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, 203851 + ... + 203957.

It is an arithmetic number, because the mean of its divisors is an integer number (516375).

Almost surely, 2^21817728 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 21817728, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (33048000).

21817728 is an abundant number, since it is smaller than the sum of its proper divisors (44278272).

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

21817728 is a wasteful number, since it uses less digits than its factorization.

21817728 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 189 (or 171 counting only the distinct ones).

The product of its digits is 12544, while the sum is 36.

The square root of 21817728 is about 4670.9450863824. The cubic root of 21817728 is about 279.4279477143.

The spelling of 21817728 in words is "twenty-one million, eight hundred seventeen thousand, seven hundred twenty-eight".