       Re: Anomaly? or at least a surprise.

• To: mathgroup at smc.vnet.net
• Subject: [mg90276] Re: [mg90250] Anomaly? or at least a surprise.
• From: "Curtis F. Osterhoudt" <cfo at lanl.gov>
• Date: Fri, 4 Jul 2008 03:57:52 -0400 (EDT)
• References: <200807031012.GAA03197@smc.vnet.net>

In my case (v. 6.0.3 on Windows XP), the second plot

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

gives a plot of ArcTan[x], NOT of the derivative w.r.t. x. This is true
whether I wrap the thing in Evaluate[] or not.

For the third plot, if you wrap it with Evaluate, then things work as
expected.

Just my two cents.

Curtis O.

> 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
>
>

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