21605243 has 4 divisors (see below), whose sum is σ = 23569368. Its totient is φ = 19641120.

The previous prime is 21605209. The next prime is 21605251. The reversal of 21605243 is 34250612.

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 21605243 - 2⁸ = 21604987 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21605263) 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, 982046 + ... + 982067.

It is an arithmetic number, because the mean of its divisors is an integer number (5892342).

Almost surely, 2^21605243 is an apocalyptic number.

21605243 is a deficient number, since it is larger than the sum of its proper divisors (1964125).

21605243 is a wasteful number, since it uses less digits than its factorization.

21605243 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1964124.

The product of its (nonzero) digits is 1440, while the sum is 23.

The square root of 21605243 is about 4648.1440382157. The cubic root of 21605243 is about 278.5178613885.

Adding to 21605243 its reverse (34250612), we get a palindrome (55855855).

The spelling of 21605243 in words is "twenty-one million, six hundred five thousand, two hundred forty-three".