21665539 has 4 divisors (see below), whose sum is σ = 24760624. Its totient is φ = 18570456.

The previous prime is 21665521. The next prime is 21665551. The reversal of 21665539 is 93556612.

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 21665539 - 2¹⁵ = 21632771 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21665498 and 21665507.

It is not an unprimeable number, because it can be changed into a prime (21665569) 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 3 ways as a sum of consecutive naturals, for example, 1547532 + ... + 1547545.

It is an arithmetic number, because the mean of its divisors is an integer number (6190156).

Almost surely, 2^21665539 is an apocalyptic number.

21665539 is a deficient number, since it is larger than the sum of its proper divisors (3095085).

21665539 is an equidigital number, since it uses as much as digits as its factorization.

21665539 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 3095084.

The product of its digits is 48600, while the sum is 37.

The square root of 21665539 is about 4654.6255488492. The cubic root of 21665539 is about 278.7767169925.

The spelling of 21665539 in words is "twenty-one million, six hundred sixty-five thousand, five hundred thirty-nine".