21218320 has 40 divisors (see below), whose sum is σ = 50543640. Its totient is φ = 8279040.

The previous prime is 21218317. The next prime is 21218321. The reversal of 21218320 is 2381212.

It can be written as a sum of positive squares in 4 ways, for example, as 1183744 + 20034576 = 1088^2 + 4476^2 .

It is a tau number, because it is divible by the number of its divisors (40).

It is not an unprimeable number, because it can be changed into a prime (21218321) 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, 46 + ... + 6514.

It is an arithmetic number, because the mean of its divisors is an integer number (1263591).

Almost surely, 2^21218320 is an apocalyptic number.

21218320 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 21218320, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (25271820).

21218320 is an abundant number, since it is smaller than the sum of its proper divisors (29325320).

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

21218320 is a wasteful number, since it uses less digits than its factorization.

21218320 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6523 (or 6517 counting only the distinct ones).

The product of its (nonzero) digits is 192, while the sum is 19.

The square root of 21218320 is about 4606.3347685551. The cubic root of 21218320 is about 276.8452001654.

Adding to 21218320 its reverse (2381212), we get a palindrome (23599532).

The spelling of 21218320 in words is "twenty-one million, two hundred eighteen thousand, three hundred twenty".