21218520 has 64 divisors (see below), whose sum is σ = 64131840. Its totient is φ = 5616000.

The previous prime is 21218513. The next prime is 21218521. The reversal of 21218520 is 2581212.

It is a happy number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21218521) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 17535 + ... + 18705.

It is an arithmetic number, because the mean of its divisors is an integer number (1002060).

Almost surely, 2^21218520 is an apocalyptic number.

21218520 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 21218520, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (32065920).

21218520 is an abundant number, since it is smaller than the sum of its proper divisors (42913320).

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

21218520 is a wasteful number, since it uses less digits than its factorization.

21218520 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1336 (or 1332 counting only the distinct ones).

The product of its (nonzero) digits is 320, while the sum is 21.

The square root of 21218520 is about 4606.3564777381. The cubic root of 21218520 is about 276.8460699934.

Adding to 21218520 its reverse (2581212), we get a palindrome (23799732).

The spelling of 21218520 in words is "twenty-one million, two hundred eighteen thousand, five hundred twenty".