66561561509 has 2 divisors, whose sum is σ = 66561561510. Its totient is φ = 66561561508.

The previous prime is 66561561499. The next prime is 66561561629. The reversal of 66561561509 is 90516516566.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 65355388609 + 1206172900 = 255647^2 + 34730^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-66561561509 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (66561561709) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 33280780754 + 33280780755.

It is an arithmetic number, because the mean of its divisors is an integer number (33280780755).

Almost surely, 2^66561561509 is an apocalyptic number.

It is an amenable number.

66561561509 is a deficient number, since it is larger than the sum of its proper divisors (1).

66561561509 is an equidigital number, since it uses as much as digits as its factorization.

66561561509 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 1458000, while the sum is 50.

The spelling of 66561561509 in words is "sixty-six billion, five hundred sixty-one million, five hundred sixty-one thousand, five hundred nine".