21365660021 has 2 divisors, whose sum is σ = 21365660022. Its totient is φ = 21365660020.

The previous prime is 21365659969. The next prime is 21365660023. The reversal of 21365660021 is 12006656312.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13808835121 + 7556824900 = 117511^2 + 86930^2 .

It is a cyclic number.

It is not a de Polignac number, because 21365660021 - 2¹⁰ = 21365658997 is a prime.

It is a Sophie Germain prime.

Together with 21365660023, it forms a pair of twin primes.

It is a Chen prime.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 21365660021.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21365660023) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10682830010 + 10682830011.

It is an arithmetic number, because the mean of its divisors is an integer number (10682830011).

Almost surely, 2^21365660021 is an apocalyptic number.

It is an amenable number.

21365660021 is a deficient number, since it is larger than the sum of its proper divisors (1).

21365660021 is an equidigital number, since it uses as much as digits as its factorization.

21365660021 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 12960, while the sum is 32.

The spelling of 21365660021 in words is "twenty-one billion, three hundred sixty-five million, six hundred sixty thousand, twenty-one".