21205605657 has 4 divisors (see below), whose sum is σ = 28274140880. Its totient is φ = 14137070436.

The previous prime is 21205605647. The next prime is 21205605679. The reversal of 21205605657 is 75650650212.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21205605657 - 2¹⁹ = 21205081369 is a prime.

It is a super-2 number, since 2×21205605657² (a number of 21 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21205605637) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (23) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3534267607 + ... + 3534267612.

It is an arithmetic number, because the mean of its divisors is an integer number (7068535220).

Almost surely, 2^21205605657 is an apocalyptic number.

It is an amenable number.

21205605657 is a deficient number, since it is larger than the sum of its proper divisors (7068535223).

21205605657 is an equidigital number, since it uses as much as digits as its factorization.

21205605657 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7068535222.

The product of its (nonzero) digits is 126000, while the sum is 39.

The spelling of 21205605657 in words is "twenty-one billion, two hundred five million, six hundred five thousand, six hundred fifty-seven".