# Matrix Calculus for Deep Learning

Hey guys! I’m currently reading Jeremy’s paper “The Matrix Calculus You Need For Deep Learning” and came across a bit I don’t understand.

In section 4.4 (Vector Sum Reduction), they are calculating the derivative of y = sum(f(x)), like so:

Here, it is noted that “Notice we were careful here to leave the parameter as a vector x
because each function fi could use all values in the vector, not just xi

What does that mean? I thought it meant that we can’t reduce fi(x) to fi(xi) to xi, but right afterwards, that is precisely what they do:

Does it have something to do with the summation being taken out? Or am I reading this completely wrong?

Here’s the link to his paper:

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They are not doing f_i(x) to f_i(x_i). It is still f_i(x) = x_i. Here he is just taking an example of f(x) =x.

I am not sure if that answers your question. If not can you describe a bit more what is the precise confusion?

Oh, yes, that answers my question xD
I thought the whole thing (including the “Notice…” part) was part of the example, and was a little confused. All good now, thanks!