21640320 has 288 divisors, whose sum is σ = 85487220. Its totient is φ = 5013504.

The previous prime is 21640279. The next prime is 21640331. The reversal of 21640320 is 2304612.

It is a happy number.

21640320 is a `hidden beast` number, since 2 + 1 + 640 + 3 + 20 = 666.

It can be written as a sum of positive squares in 6 ways, for example, as 11451456 + 10188864 = 3384^2 + 3192^2 .

It is a tau number, because it is divible by the number of its divisors (288).

It is a Harshad number since it is a multiple of its sum of digits (18).

It is a junction number, because it is equal to n+sod(n) for n = 21640293 and 21640302.

It is an unprimeable number.

It is a polite number, since it can be written in 35 ways as a sum of consecutive naturals, for example, 1272952 + ... + 1272968.

Almost surely, 2^21640320 is an apocalyptic number.

21640320 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 21640320, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (42743610).

21640320 is an abundant number, since it is smaller than the sum of its proper divisors (63846900).

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

21640320 is a wasteful number, since it uses less digits than its factorization.

21640320 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 72 (or 40 counting only the distinct ones).

The product of its (nonzero) digits is 288, while the sum is 18.

The square root of 21640320 is about 4651.9157344045. The cubic root of 21640320 is about 278.6685082892.

Adding to 21640320 its reverse (2304612), we get a palindrome (23944932).

The spelling of 21640320 in words is "twenty-one million, six hundred forty thousand, three hundred twenty".