double number represention

https://math.stackexchange.com/questions/1349073/how-to-find-out-the-number-of-repeating-digits-of-a-rational-number-in-decimal-f

https://oeis.org/A007732

https://www.wikihow.com/Convert-Repeating-Decimals-to-Fractions

对于巨大数字的除法很有用处。同时，考虑fractional number representation。这将消除舍入误差。