Plot, Cursor and Spelling Errors questions

• To: mathgroup at smc.vnet.net
• Subject: [mg14393] Plot, Cursor and Spelling Errors questions
• From: Ranko Bojanic <bojanic at math.ohio-state.edu>
• Date: Sun, 18 Oct 1998 15:09:59 -0400
• Organization: Ohio State University
• Sender: owner-wri-mathgroup at wolfram.com

Dear Mathematica friends:

I am having problems when trying to plot the graph of an error curve
when the magnitude of the error curve is close to the \$MachineEpsilon
number. The following example is typical:

n = 15;
f[x_] := Exp[x];
xLst = Table[Cos[((2*k - 1)*Pi)/(2*n)], {k, 1, n}]//Reverse; yLst =
N[Map[f,xLst], 30];
fLst = Transpose[{xLst, yLst}];
p[x_] := InterpolatingPolynomial[fLst, x]; Plot[Evaluate[f[x]-P[x]], {x,
-1, 1}, PlotPoints -> 50];

Why the Plot function cannot draw the graph of the difference? I can
still get a graph by computing 1000 points on the error curve and using
ListPlot with PlotJoined->True, but the graph is not very smooth.

The problem here is not in the accuracy of computations, as the ListPlot
function shows, but in the way Plot processes data. Is there a way to
force Plot to draw the error curve correctly?

An unrelated programming question:While  Mathematica is doing a lengthy
computation, I would like to change the cursor into a watch cursor. I
could not find any information on how to do it. Is there a simple
command to change the current cursor into a watch cursor - something
like SetCursor and InitCursor in Pascal.

Last question: If you run the program listed here, you will probably get
the messages :
General::spell:Possible spelling error: new symbol "yLst" is similar
to existing symbol {xLst} General::spell:Possible
spelling error: new symbol "fLst" is similar
to existing symbol {xLst,yLst}
Is there a way to turn off these spelling errors messages?

Thanks,
Ranko Bojanic
bojanic at math.ohio-state.edu

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