21205905 has 16 divisors (see below), whose sum is σ = 38776704. Its totient is φ = 9694080.

The previous prime is 21205901. The next prime is 21205909. The reversal of 21205905 is 50950212.

It is an interprime number because it is at equal distance from previous prime (21205901) and next prime (21205909).

It is not a de Polignac number, because 21205905 - 2² = 21205901 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21205901) 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 15 ways as a sum of consecutive naturals, for example, 100876 + ... + 101085.

It is an arithmetic number, because the mean of its divisors is an integer number (2423544).

Almost surely, 2^21205905 is an apocalyptic number.

It is an amenable number.

21205905 is a deficient number, since it is larger than the sum of its proper divisors (17570799).

21205905 is a wasteful number, since it uses less digits than its factorization.

21205905 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 201976.

The product of its (nonzero) digits is 900, while the sum is 24.

The square root of 21205905 is about 4604.9869706656. The cubic root of 21205905 is about 276.7911948888.

The spelling of 21205905 in words is "twenty-one million, two hundred five thousand, nine hundred five".