Hi I was trying to figure out how to divide values from a DataFrame. But here I made an example for pandas series

```
a = pd.Series([1, 2, 16,64,128,360,720])
a
-----------------
0     1
1     2
2     16
3     64
4    128
5    360
6    720

```

So is there any way I could divide a number in a given row by the value from the previous row?

```
0     2
1     8
2     4
3     2
4     2.8
5     2

```

Furthermore, I also tried to get the output like "if the value is double, print the index".

Thank you for your help!