21728720 has 80 divisors (see below), whose sum is σ = 57652560. Its totient is φ = 7544832.

The previous prime is 21728719. The next prime is 21728741. The reversal of 21728720 is 2782712.

It can be written as a sum of positive squares in 8 ways, for example, as 1008016 + 20720704 = 1004^2 + 4552^2 .

It is a tau number, because it is divible by the number of its divisors (80).

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 17066 + ... + 18294.

It is an arithmetic number, because the mean of its divisors is an integer number (720657).

Almost surely, 2^21728720 is an apocalyptic number.

21728720 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 21728720, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (28826280).

21728720 is an abundant number, since it is smaller than the sum of its proper divisors (35923840).

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

21728720 is a wasteful number, since it uses less digits than its factorization.

21728720 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1272 (or 1266 counting only the distinct ones).

The product of its (nonzero) digits is 3136, while the sum is 29.

The square root of 21728720 is about 4661.4075127584. The cubic root of 21728720 is about 279.0474433607.

The spelling of 21728720 in words is "twenty-one million, seven hundred twenty-eight thousand, seven hundred twenty".