symbol correspondence issue

[LucasSun2002]

In Fig. 3, the issue arises because the subscript i is used in both the second and third parts to represent different things, potentially causing a mismatch in interpretation. Specifically:
In the third part, when discussing f_{d}^{4}(), the subscript i is set to 4.
However, in the second part, the feature input also uses f_{d}^{4}(), where the subscript i should still be 4.
This overlap in the use of i creates confusion in the correspondence between these two sections. It might be better to distinguish between these two uses of i to avoid ambiguity in the figure.