21605309309 has 2 divisors, whose sum is σ = 21605309310. Its totient is φ = 21605309308.

The previous prime is 21605309303. The next prime is 21605309329. The reversal of 21605309309 is 90390350612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15137611225 + 6467698084 = 123035^2 + 80422^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21605309309 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21605309303) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10802654654 + 10802654655.

It is an arithmetic number, because the mean of its divisors is an integer number (10802654655).

Almost surely, 2^21605309309 is an apocalyptic number.

It is an amenable number.

21605309309 is a deficient number, since it is larger than the sum of its proper divisors (1).

21605309309 is an equidigital number, since it uses as much as digits as its factorization.

21605309309 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 43740, while the sum is 38.

The spelling of 21605309309 in words is "twenty-one billion, six hundred five million, three hundred nine thousand, three hundred nine".