21906805 has 4 divisors (see below), whose sum is σ = 26288172. Its totient is φ = 17525440.

The previous prime is 21906767. The next prime is 21906809. The reversal of 21906805 is 50860912.

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 5447556 + 16459249 = 2334^2 + 4057^2 .

It is not a de Polignac number, because 21906805 - 2¹⁵ = 21874037 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 (21906809) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2190676 + ... + 2190685.

It is an arithmetic number, because the mean of its divisors is an integer number (6572043).

Almost surely, 2^21906805 is an apocalyptic number.

It is an amenable number.

21906805 is a deficient number, since it is larger than the sum of its proper divisors (4381367).

21906805 is an equidigital number, since it uses as much as digits as its factorization.

21906805 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4381366.

The product of its (nonzero) digits is 4320, while the sum is 31.

The square root of 21906805 is about 4680.4705959978. The cubic root of 21906805 is about 279.8077123661.

The spelling of 21906805 in words is "twenty-one million, nine hundred six thousand, eight hundred five".