21005566 has 8 divisors (see below), whose sum is σ = 32360040. Its totient is φ = 10218888.

The previous prime is 21005561. The next prime is 21005597. The reversal of 21005566 is 66550012.

It is a sphenic number, since it is the product of 3 distinct primes.

It is a self number, because there is not a number n which added to its sum of digits gives 21005566.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21005561) 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 3 ways as a sum of consecutive naturals, for example, 141856 + ... + 142003.

It is an arithmetic number, because the mean of its divisors is an integer number (4045005).

Almost surely, 2^21005566 is an apocalyptic number.

21005566 is a deficient number, since it is larger than the sum of its proper divisors (11354474).

21005566 is a wasteful number, since it uses less digits than its factorization.

21005566 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 283898.

The product of its (nonzero) digits is 1800, while the sum is 25.

The square root of 21005566 is about 4583.1829551088. The cubic root of 21005566 is about 275.9167903611.

Adding to 21005566 its reverse (66550012), we get a palindrome (87555578).

The spelling of 21005566 in words is "twenty-one million, five thousand, five hundred sixty-six".