21215540 has 12 divisors (see below), whose sum is σ = 44552676. Its totient is φ = 8486208.

The previous prime is 21215539. The next prime is 21215543. The reversal of 21215540 is 4551212.

It can be written as a sum of positive squares in 2 ways, for example, as 16224784 + 4990756 = 4028^2 + 2234^2 .

It is a Harshad number since it is a multiple of its sum of digits (20), and also a Moran number because the ratio is a prime number: 1060777 = 21215540 / (2 + 1 + 2 + 1 + 5 + 5 + 4 + 0).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21215543) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 530369 + ... + 530408.

It is an arithmetic number, because the mean of its divisors is an integer number (3712723).

Almost surely, 2^21215540 is an apocalyptic number.

21215540 is a gapful number since it is divisible by the number (20) formed by its first and last digit.

It is an amenable number.

21215540 is an abundant number, since it is smaller than the sum of its proper divisors (23337136).

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

21215540 is a wasteful number, since it uses less digits than its factorization.

21215540 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1060786 (or 1060784 counting only the distinct ones).

The product of its (nonzero) digits is 400, while the sum is 20.

The square root of 21215540 is about 4606.0330003160. The cubic root of 21215540 is about 276.8331089902.

Adding to 21215540 its reverse (4551212), we get a palindrome (25766752).

The spelling of 21215540 in words is "twenty-one million, two hundred fifteen thousand, five hundred forty".