21305331665 has 4 divisors (see below), whose sum is σ = 25566398004. Its totient is φ = 17044265328.

The previous prime is 21305331631. The next prime is 21305331667. The reversal of 21305331665 is 56613350312.

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 2168206096 + 19137125569 = 46564^2 + 138337^2 .

It is a cyclic number.

It is not a de Polignac number, because 21305331665 - 2¹⁸ = 21305069521 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21305331667) 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, 2130533162 + ... + 2130533171.

It is an arithmetic number, because the mean of its divisors is an integer number (6391599501).

Almost surely, 2^21305331665 is an apocalyptic number.

It is an amenable number.

21305331665 is a deficient number, since it is larger than the sum of its proper divisors (4261066339).

21305331665 is an equidigital number, since it uses as much as digits as its factorization.

21305331665 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 4261066338.

The product of its (nonzero) digits is 48600, while the sum is 35.

Adding to 21305331665 its reverse (56613350312), we get a palindrome (77918681977).

The spelling of 21305331665 in words is "twenty-one billion, three hundred five million, three hundred thirty-one thousand, six hundred sixty-five".