21205505 has 4 divisors (see below), whose sum is σ = 25446612. Its totient is φ = 16964400.

The previous prime is 21205453. The next prime is 21205523. The reversal of 21205505 is 50550212.

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 2534464 + 18671041 = 1592^2 + 4321^2 .

It is not a de Polignac number, because 21205505 - 2¹⁸ = 20943361 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2120546 + ... + 2120555.

It is an arithmetic number, because the mean of its divisors is an integer number (6361653).

Almost surely, 2^21205505 is an apocalyptic number.

It is an amenable number.

21205505 is a deficient number, since it is larger than the sum of its proper divisors (4241107).

21205505 is an equidigital number, since it uses as much as digits as its factorization.

21205505 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4241106.

The product of its (nonzero) digits is 500, while the sum is 20.

The square root of 21205505 is about 4604.9435392847. The cubic root of 21205505 is about 276.7894545374.

Adding to 21205505 its reverse (50550212), we get a palindrome (71755717).

The spelling of 21205505 in words is "twenty-one million, two hundred five thousand, five hundred five".