21605566 has 4 divisors (see below), whose sum is σ = 32408352. Its totient is φ = 10802782.

The previous prime is 21605561. The next prime is 21605567. The reversal of 21605566 is 66550612.

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

It is a Smith number, since the sum of its digits (31) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (21605561) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 5401390 + ... + 5401393.

It is an arithmetic number, because the mean of its divisors is an integer number (8102088).

Almost surely, 2^21605566 is an apocalyptic number.

21605566 is a deficient number, since it is larger than the sum of its proper divisors (10802786).

21605566 is a wasteful number, since it uses less digits than its factorization.

21605566 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 10802785.

The product of its (nonzero) digits is 10800, while the sum is 31.

The square root of 21605566 is about 4648.1787831365. The cubic root of 21605566 is about 278.5192493359.

The spelling of 21605566 in words is "twenty-one million, six hundred five thousand, five hundred sixty-six".