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I do not understand why these parts give different results. Part 1. > restart; > u:=(t,x)->sum(a[n](t)*sin(n*Pi*x),n=1..2); > diff(u(t,x),t); > D[1](u)(t,x); THE OUTPUT OF THE LAST LINE IS WHAT A HUMAN WOULD GET. Part 2. > restart; > N:=2; > u:=(t,x)->sum(a[n](t)*sin(n*Pi*x),n=1..N); > diff(u(t,x),t); > D[1](u)(t,x); THE OUTPUT OF THE LAST LINE IS 0, curiously. Part 3. > restart; > N:=2; > u:=(t,x)->add(a[n](t)*sin(n*Pi*x),n=1..N); > diff(u(t,x),t); > D[1](u)(t,x); THE OUTPUT OF THE LAST LINE ECHOES THE REQUEST. > I prefer the output of Part 1, of course. I can tolerate the output of Part 3. But, the output of Part 2 seems wrong. What am I not understanding? Jim Herod >

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