On the GLM chapter and on one of the answers it goes from: ln L (mu_ij) = sum over i sum over j (- mu_ij + y_ij ln mu_ij) + const Then: d ln L/d mu_ij = -1 + y_ij/mu_ij = 0. Why have the summations been ignored? I was trying to put some values of i and j in there to get rid of the summations but I got confused with what it means. Indeed in the next question it's a very similar thing but with no ij subscripts on the mu but they keep the summations. Anyone help please? (Apr 2004 question 9)
Suppose we consider mu13 - only one of all the 12 summations has this particular term - so when we differentiate with respect to mu13 we'll only get that particular part of the summation. Whereas mu is in all of the 12 summations - so they stay when we differentiate. Does that help?
