21218539 has 4 divisors (see below), whose sum is σ = 21903040. Its totient is φ = 20534040.

The previous prime is 21218521. The next prime is 21218551. The reversal of 21218539 is 93581212.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 21218539 - 2⁵ = 21218507 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (31), and also a Moran number because the ratio is a prime number: 684469 = 21218539 / (2 + 1 + 2 + 1 + 8 + 5 + 3 + 9).

It is a Duffinian number.

It is a self number, because there is not a number n which added to its sum of digits gives 21218539.

It is not an unprimeable number, because it can be changed into a prime (21218599) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 342204 + ... + 342265.

It is an arithmetic number, because the mean of its divisors is an integer number (5475760).

Almost surely, 2^21218539 is an apocalyptic number.

21218539 is a deficient number, since it is larger than the sum of its proper divisors (684501).

21218539 is an equidigital number, since it uses as much as digits as its factorization.

21218539 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 684500.

The product of its digits is 4320, while the sum is 31.

The square root of 21218539 is about 4606.3585401052. The cubic root of 21218539 is about 276.8461526267.

The spelling of 21218539 in words is "twenty-one million, two hundred eighteen thousand, five hundred thirty-nine".