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plot2d assumes (unless told otherwise) that the expression

to be plotted can be evaluated at any point within [1, 10].

However sum(1/i,i,1,n) is only well-defined for integer n.

Perhaps you meant

L1 : makelist (n, n, 1, 10);

L2 : makelist (sum(1/i, i, 1, n), n, 1, 10);

L2 : L2, numer;

plot2d ([discrete, L1, L2]);

Although "plot2d(sum(1/i,i,1,n), [n,1,10]); isn't valid,

plot2d should try to do something smarter than stack

overflow in that case.