15645565721 has 2 divisors, whose sum is σ = 15645565722. Its totient is φ = 15645565720.

The previous prime is 15645565711. The next prime is 15645565807. The reversal of 15645565721 is 12756554651.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15559818121 + 85747600 = 124739^2 + 9260^2 .

It is a cyclic number.

It is not a de Polignac number, because 15645565721 - 2⁶ = 15645565657 is a prime.

It is not a weakly prime, because it can be changed into another prime (15645565711) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 7822782860 + 7822782861.

It is an arithmetic number, because the mean of its divisors is an integer number (7822782861).

Almost surely, 2^15645565721 is an apocalyptic number.

It is an amenable number.

15645565721 is a deficient number, since it is larger than the sum of its proper divisors (1).

15645565721 is an equidigital number, since it uses as much as digits as its factorization.

15645565721 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1260000, while the sum is 47.

The spelling of 15645565721 in words is "fifteen billion, six hundred forty-five million, five hundred sixty-five thousand, seven hundred twenty-one".