We get a bunch of consecutive values
x | y
1 | -1
2 | 1
3 | 3
4 | 5
5 | 7
6 | 9
. | .
. | .
. | .
We observe
1. That we start with the first term a1=-1
We also observe
2. That we add 2 to each term to get the next term.
Therefore, the recursion equation is
a1 = -1, an = an-1 + 2 for n > 2
Edwin