21809105 has 4 divisors (see below), whose sum is σ = 26170932. Its totient is φ = 17447280.

The previous prime is 21809101. The next prime is 21809111. The reversal of 21809105 is 50190812.

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 6723649 + 15085456 = 2593^2 + 3884^2 .

It is not a de Polignac number, because 21809105 - 2² = 21809101 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21809101) 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, 2180906 + ... + 2180915.

It is an arithmetic number, because the mean of its divisors is an integer number (6542733).

Almost surely, 2^21809105 is an apocalyptic number.

It is an amenable number.

21809105 is a deficient number, since it is larger than the sum of its proper divisors (4361827).

21809105 is an equidigital number, since it uses as much as digits as its factorization.

21809105 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4361826.

The product of its (nonzero) digits is 720, while the sum is 26.

The square root of 21809105 is about 4670.0219485566. The cubic root of 21809105 is about 279.3911301822.

Adding to 21809105 its reverse (50190812), we get a palindrome (71999917).

The spelling of 21809105 in words is "twenty-one million, eight hundred nine thousand, one hundred five".