21205605647 has 2 divisors, whose sum is σ = 21205605648. Its totient is φ = 21205605646.

The previous prime is 21205605637. The next prime is 21205605679. The reversal of 21205605647 is 74650650212.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 21205605647 - 2¹⁸ = 21205343503 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21205605647.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21205605637) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10602802823 + 10602802824.

It is an arithmetic number, because the mean of its divisors is an integer number (10602802824).

Almost surely, 2^21205605647 is an apocalyptic number.

21205605647 is a deficient number, since it is larger than the sum of its proper divisors (1).

21205605647 is an equidigital number, since it uses as much as digits as its factorization.

21205605647 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 100800, while the sum is 38.

The spelling of 21205605647 in words is "twenty-one billion, two hundred five million, six hundred five thousand, six hundred forty-seven".