calculus and analysis - How to express and take symbolic derivative of expression with finite sum

I have the expression:

$$ p_i = \frac{e^{x_i}}{\sum_{j=1}^{n} e^{x_j}} $$

which I defined with the following Mathematica code:

Subscript[p, i] = E^Subscript[x, i] / Sum[E^Subscript[x, j], {j, 1, n}]

I tried to take the derivative with respect to an $x_i$:

D[Subscript[p, i], Subscript[x,i]]

but, I just get the expression back as the result:

$$ \frac{e^{x_i}}{\sum_{j=1}^{n} e^{x_j}} $$

Obviously, I am doing something wrong. How should I go about working with this type of expression?

Comments

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