21205105 has 4 divisors (see below), whose sum is σ = 25446132. Its totient is φ = 16964080.

The previous prime is 21205099. The next prime is 21205109. The reversal of 21205105 is 50150212.

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 3489424 + 17715681 = 1868^2 + 4209^2 .

It is not a de Polignac number, because 21205105 - 2¹⁵ = 21172337 is a prime.

It is a super-2 number, since 2×21205105² = 899312956122050, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21205109) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2120506 + ... + 2120515.

It is an arithmetic number, because the mean of its divisors is an integer number (6361533).

Almost surely, 2^21205105 is an apocalyptic number.

It is an amenable number.

21205105 is a deficient number, since it is larger than the sum of its proper divisors (4241027).

21205105 is an equidigital number, since it uses as much as digits as its factorization.

21205105 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4241026.

The product of its (nonzero) digits is 100, while the sum is 16.

The square root of 21205105 is about 4604.9001074942. The cubic root of 21205105 is about 276.7877141642.

Adding to 21205105 its reverse (50150212), we get a palindrome (71355317).

It can be divided in two parts, 2120 and 5105, that added together give a square (7225 = 85²).

The spelling of 21205105 in words is "twenty-one million, two hundred five thousand, one hundred five".