21664305665 has 4 divisors (see below), whose sum is σ = 25997166804. Its totient is φ = 17331444528.

The previous prime is 21664305661. The next prime is 21664305683. The reversal of 21664305665 is 56650346612.

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 7317775936 + 14346529729 = 85544^2 + 119777^2 .

It is a cyclic number.

It is not a de Polignac number, because 21664305665 - 2² = 21664305661 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21664305661) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2166430562 + ... + 2166430571.

It is an arithmetic number, because the mean of its divisors is an integer number (6499291701).

Almost surely, 2^21664305665 is an apocalyptic number.

It is an amenable number.

21664305665 is a deficient number, since it is larger than the sum of its proper divisors (4332861139).

21664305665 is an equidigital number, since it uses as much as digits as its factorization.

21664305665 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4332861138.

The product of its (nonzero) digits is 777600, while the sum is 44.

The spelling of 21664305665 in words is "twenty-one billion, six hundred sixty-four million, three hundred five thousand, six hundred sixty-five".