21205906 has 4 divisors (see below), whose sum is σ = 31808862. Its totient is φ = 10602952.

The previous prime is 21205901. The next prime is 21205909. The reversal of 21205906 is 60950212.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in only one way, i.e., 20620681 + 585225 = 4541^2 + 765^2 .

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 as a sum of consecutive naturals, namely, 5301475 + ... + 5301478.

Almost surely, 2^21205906 is an apocalyptic number.

21205906 is a deficient number, since it is larger than the sum of its proper divisors (10602956).

21205906 is a wasteful number, since it uses less digits than its factorization.

21205906 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10602955.

The product of its (nonzero) digits is 1080, while the sum is 25.

The square root of 21205906 is about 4604.9870792435. The cubic root of 21205906 is about 276.7911992396.

The spelling of 21205906 in words is "twenty-one million, two hundred five thousand, nine hundred six".