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Re: Dealing with Indeterminant points & ListPlot

• To: mathgroup at smc.vnet.net
• Subject: [mg2076] Re: Dealing with Indeterminant points & ListPlot
• From: David Harvatin <dtharvat at sprint.uccs.edu>
• Date: Sat, 23 Sep 1995 20:36:38 -0400
• Organization: University of Colorado at Boulder

Ed Oswalt <efo at pixel.kodak.com> wrote:
>I am using the latest Student version of Mathematica and was having a 
>problem dealing with indeterminant points in in a function like
>Sin[x]/x where Sin is indeterminant at x = 0 (but by L'Hopitals is
>equal to 1. When I try to plot such functions Mathematica complains.
>
>What can I do to get around this? Can I use an If[] function inside
>other functions like Plot[] to test for x = 0 and sub in the value
>1 for the formula in that instance? How do I do that - tried and
>can't get it to work right.
>
>I just started with Mathematica this weekend and was a little
>dissapointed since my HP-48GX calculator handles these functions fine
>(not sure if attempts L'Hopital's or just follows the convergance to
>1).  I plan to e-mail them the suggestion to handle these points better
>(even if it means a switch to activativate finding a symbolic
>L'Hopitals solution or just following where the convergance appears to
>be at.
>
>Also I am using ListPlot[] on the same student windows version and the dots
>are too small - they are invisable on the screen and barely visable on 
>print out, I end up having to join the points so I can see them but that is 
>not what I want to show - I want to show the points. Any Idea's on how to
>make these dots "Fatter"?
>
>Thanks for any help,
>
>Ed Oswalt 
>Part time student & Full Time Engineer
>efo at dtc.kodak.com
>
>
>
>

Ed,

In response to your first question regarding the sinc function : For plotting and other 
numerical purposes use sinc[x_] = If[x != 0, Sin[x]/x, 1].  If you try to integrate this new 
function, MMA objects to the discontinuity at x = 0.  In this case, I would recommend a set of 
signal processing packages written by Brian Evans, where he has defined a sinc function that 
MMA can deal with in integration.  I don't know how he did it, but you can check his code.  He 
uses the name "Sinc".  The packages are freely available on MathSource (FTP : 
mathsource.wri.com), and the PC version of the file is called "SigProc2_IBM_PC.zip".  Be sure 
to follow his installation instructions to the letter.

In response to your second question, make sure that the "Make Lines Thin" option under the 
Graph menu does not have a check mark by it, and make sure that "Show Lines" does have a check 
mark by it.  Then enlarge the graphic by dragging with the mouse.  I have found this to help in 
some cases with the ListPlot command.  A better solution might be to use the SetOptions, 
PlotStyle, and PointSize commands.  Good luck.

Dave Harvatin
dtharvat at sprint.uccs.edu