   x   y  forward  backward  central  exact
0  0   0      1.0       NaN      NaN      0
1  1   1      3.0       1.0      2.0      2
2  2   4      5.0       3.0      4.0      4
3  3   9      7.0       5.0      6.0      6
4  4  16      NaN       7.0      NaN      8
