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>*Reply-to*: cfo at lanl.gov

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 > >

**References**:**Anomaly? or at least a surprise.***From:*Steve Gray <stevebg@roadrunner.com>