Re: Anomaly? or at least a surprise.

*To*: mathgroup at smc.vnet.net*Subject*: [mg90297] Re: [mg90250] Anomaly? or at least a surprise.*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Fri, 4 Jul 2008 04:02:01 -0400 (EDT)*Reply-to*: hanlonr at cox.net

Plot[D[ArcTan[x]], {x, -10, 10}, PlotRange -> Full] gives the Plot of ArcTan[x], i.e., without the variable the D had no effect. The Plot you wanted would be Plot[Evaluate[D[ArcTan[x], x]], {x, -10, 10}, PlotRange -> Full] Bob Hanlon ---- Steve Gray <stevebg at roadrunner.com> 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