21605905 has 4 divisors (see below), whose sum is σ = 25927092. Its totient is φ = 17284720.

The previous prime is 21605897. The next prime is 21605911. The reversal of 21605905 is 50950612.

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 3932289 + 17673616 = 1983^2 + 4204^2 .

It is not a de Polignac number, because 21605905 - 2³ = 21605897 is a prime.

It is a Duffinian number.

It is an unprimeable number.

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, 2160586 + ... + 2160595.

It is an arithmetic number, because the mean of its divisors is an integer number (6481773).

Almost surely, 2^21605905 is an apocalyptic number.

It is an amenable number.

21605905 is a deficient number, since it is larger than the sum of its proper divisors (4321187).

21605905 is an equidigital number, since it uses as much as digits as its factorization.

21605905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4321186.

The product of its (nonzero) digits is 2700, while the sum is 28.

The square root of 21605905 is about 4648.2152488885. The cubic root of 21605905 is about 278.5207060212.

The spelling of 21605905 in words is "twenty-one million, six hundred five thousand, nine hundred five".