21636805 has 4 divisors (see below), whose sum is σ = 25964172. Its totient is φ = 17309440.

The previous prime is 21636803. The next prime is 21636809. The reversal of 21636805 is 50863612.

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 17048641 + 4588164 = 4129^2 + 2142^2 .

It is not a de Polignac number, because 21636805 - 2¹ = 21636803 is a prime.

It is a Smith number, since the sum of its digits (31) 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 a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21636803) 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, 2163676 + ... + 2163685.

It is an arithmetic number, because the mean of its divisors is an integer number (6491043).

Almost surely, 2^21636805 is an apocalyptic number.

It is an amenable number.

21636805 is a deficient number, since it is larger than the sum of its proper divisors (4327367).

21636805 is an equidigital number, since it uses as much as digits as its factorization.

21636805 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4327366.

The product of its (nonzero) digits is 8640, while the sum is 31.

The square root of 21636805 is about 4651.5379177214. The cubic root of 21636805 is about 278.6534195899.

The spelling of 21636805 in words is "twenty-one million, six hundred thirty-six thousand, eight hundred five".