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Re: Anomaly? or at least a surprise.


On 3 Jul 2008, at 19:12, Steve Gray wrote:

> Doing D[ArcTan[x], x] gives 1/(x^2+1) as expected.
> Doing Plot[D[ArcTan[x]],{x,-10,10},PlotRange->Full] with no variable
> to differentiate by gives a correct plot (!) This is surprising - Help
> says nothing about leaving out the independent variable even when the
> function is of only one variable. Now, doing
> Plot[D[ArcTan[x], x], {x, -10, 10}, PlotRange -> Full]
> gives error messages such as
>
> General::ivar: -9.18326 is not a valid variable. >>
> General::ivar: -9.59143 is not a valid variable. >>, etc.
>
> I thought that D[ArcTan[x], x] and 1/(x^2+1) should behave identically
> in the Plot statement.
>
> Is this behavior something I should know? Thank you.
>
> Steve Gray
>

I don't think this

Plot[D[ArcTan[x]],{x,-10,10},PlotRange->Full]

is "the correct plot" of this:

Plot[Evaluate[D[ArcTan[x], x]], {x, -10, 10}, PlotRange -> Full]

Andrzej Kozlowski





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