21605305 has 4 divisors (see below), whose sum is σ = 25926372. Its totient is φ = 17284240.

The previous prime is 21605293. The next prime is 21605321. The reversal of 21605305 is 50350612.

It is a happy number.

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

It can be written as a sum of positive squares in 2 ways, for example, as 12020089 + 9585216 = 3467^2 + 3096^2 .

It is not a de Polignac number, because 21605305 - 2¹³ = 21597113 is a prime.

It is a Smith number, since the sum of its digits (22) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2160526 + ... + 2160535.

It is an arithmetic number, because the mean of its divisors is an integer number (6481593).

Almost surely, 2^21605305 is an apocalyptic number.

It is an amenable number.

21605305 is a deficient number, since it is larger than the sum of its proper divisors (4321067).

21605305 is an equidigital number, since it uses as much as digits as its factorization.

21605305 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4321066.

The product of its (nonzero) digits is 900, while the sum is 22.

The square root of 21605305 is about 4648.1507075395. The cubic root of 21605305 is about 278.5181278067.

Adding to 21605305 its reverse (50350612), we get a palindrome (71955917).

The spelling of 21605305 in words is "twenty-one million, six hundred five thousand, three hundred five".