subst() vs x=a with diff()

08292019, 05:05 PM
Post: #1




subst() vs x=a with diff()
I ran into an issue that I mostly sorted out and thought others might benefit from the experience.
I was attempting to find a 2nd derivative at a point so I tried diff(t*e^t,t,2)t=3 but the t=3 is substituted first, so I got diff(3*e^3,3,2) I was able to use the following work around ∂(∂(t*e^t,t),t)t=3 but it's not exactly elegant (looks even worse in Textbook display), and it doesn't scale easily to higher order derivatives. The problem is the t=3 substitution occurs before the diff() command is run. I found that the subst() command makes the substitution after its expression is evaluated, so this works as intended. subst(diff(t*e^t,t,2),t,3) Since something like ∂(sin(x),x)x=2 does work, I guess that ∂() has a higher evaluation order than x=2. Seems like diff() should be given this same behavior. 

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