21605239 has 4 divisors (see below), whose sum is σ = 21615160. Its totient is φ = 21595320.

The previous prime is 21605209. The next prime is 21605251. The reversal of 21605239 is 93250612.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It is a cyclic number.

It is not a de Polignac number, because 21605239 - 2⁹ = 21604727 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21605209) 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, 117 + ... + 6574.

It is an arithmetic number, because the mean of its divisors is an integer number (5403790).

Almost surely, 2^21605239 is an apocalyptic number.

21605239 is a deficient number, since it is larger than the sum of its proper divisors (9921).

21605239 is an equidigital number, since it uses as much as digits as its factorization.

21605239 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 9920.

The product of its (nonzero) digits is 3240, while the sum is 28.

The square root of 21605239 is about 4648.1436079364. The cubic root of 21605239 is about 278.5178442002.

The spelling of 21605239 in words is "twenty-one million, six hundred five thousand, two hundred thirty-nine".