0
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If I define

f[x_, j_] = D[E^x^2, {x, j}];

and then evaluate

f[0, 2]

I get the error

General::ivar: 0 is not a valid variable.

How can I tell Mathematica to evaluate the derivative first before setting $x = 0$?

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3
  • $\begingroup$ You could use Derivative[2][E^#^2 &][0] or more generally Derivative[2][E^#^2 &][x]. $\endgroup$ Commented May 25, 2017 at 15:00
  • $\begingroup$ @b.gatessucks Just tested this. For some reason this increases evaluation time almost 5x. Any ideas what might be causing this? $\endgroup$
    – Janosh
    Commented May 25, 2017 at 15:29
  • $\begingroup$ Related: mathematica.stackexchange.com/questions/20217/… $\endgroup$
    – Michael E2
    Commented Oct 25, 2019 at 10:47

1 Answer 1

1
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Use Rules :

In[18]:= f[x1_, j_] := D[E^x^2, {x, j}] /. x -> x1

In[19]:= f[0, 2]

Out[19]= 2
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1
  • 3
    $\begingroup$ It is better to localize dummy variable, i.e. f[x_, j_] := Module[{dummy}, D[E^dummy^2, {dummy, j}] /. dummy->x], otherwise one can get an error if x is defined somewhere before. $\endgroup$
    – Ray Shadow
    Commented May 25, 2017 at 11:28

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