We will use F2 to mean the Fibonacci number in the table next to the 2 in the first column,
so F5 refers to the Fibonacci number next to the 5 in the first column,
as you can see F4=3 and F7=13.
n Fn
0 0
1 1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19

introduction to the FIBONACCI numbers