21205918009 has 2 divisors, whose sum is σ = 21205918010. Its totient is φ = 21205918008.

The previous prime is 21205917989. The next prime is 21205918057. The reversal of 21205918009 is 90081950212.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 14557146409 + 6648771600 = 120653^2 + 81540^2 .

It is a cyclic number.

It is not a de Polignac number, because 21205918009 - 2¹⁵ = 21205885241 is a prime.

It is not a weakly prime, because it can be changed into another prime (21205913009) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10602959004 + 10602959005.

It is an arithmetic number, because the mean of its divisors is an integer number (10602959005).

Almost surely, 2^21205918009 is an apocalyptic number.

It is an amenable number.

21205918009 is a deficient number, since it is larger than the sum of its proper divisors (1).

21205918009 is an equidigital number, since it uses as much as digits as its factorization.

21205918009 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 12960, while the sum is 37.

The spelling of 21205918009 in words is "twenty-one billion, two hundred five million, nine hundred eighteen thousand, nine".