I have the following code

ExD = Sum[D[y[n, t], t], {n, 0, 1}];
ExS = Sum[y[n, t], {n, 0, 1}];
σ[0] = 1;
σ'[0] = 1;
y[0, t_] = σ[0] + t σ'[0]

Note that when executed, ExD is still general for D[y[0,t],t], while ExS prints y[0,t] as it was defined. I cannot seem to figure out why the first derivative with respect to the second variable of y[0,t] does not evaluate even though y[0,t] is clearly defined and prints properly. Do I need to use Evaluate[] here?


1 Answer 1


I just realized if I use := here instead of = for ExD it will evaluate the derivative every time D[y[0,t],t] is called. This solves my problem.

  • $\begingroup$ Maybe you want ExD = Sum[Evaluate@D[y[n, t], t], {n, 0, 1}]? $\endgroup$ Mar 10, 2020 at 19:28

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