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MathGroup Archive 2006

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Re: Possible Bug in ArcTan ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg64797] Re: [mg64770] Possible Bug in ArcTan ?
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Thu, 2 Mar 2006 21:46:52 -0500 (EST)
  • Reply-to: hanlonr at cox.net
  • Sender: owner-wri-mathgroup at wolfram.com

Plot3D[ArcTan[y/x], {x, -1, 1}, {y, -1, 1}];

As expected, there is a discontinuity for x=0

Plot[Evaluate[Table[ArcTan[y/x],
        {x,-1+10^-8,1+10^-8,1/2}]],{y,-1,1},
    PlotStyle->Table[Hue[(x+3)/4],{x,-1,1,1/2}],
    Frame->True,Axes->False];


Bob Hanlon

> 
> From: "JCDenton" <post4uw at web.de>
To: mathgroup at smc.vnet.net
> Subject: [mg64797] [mg64770] Possible Bug in ArcTan ?
> 
> Hi Group!
> 
> I am describing resonant electrical systems with Mathematica. So there
> is a lot of dealing with complex numbers and functions. One important
> function is:
> 
> phi=ArcTan[Im/Re]
> 
> This function is zero when Im = 0. The problem is, that when I plot
> this function it shows some strange behaviour near this zero point. It
> looks somehow, as if the function values are multiplied with -1 or
> something like that. I also tried to calculate a table an plot this
> with gnuplot where the stepsize was 0.005. I was the same strange
> situation. So I am sure it is not the Plot function that is buggy but
> the ArcTan itself. I also tried the table with step size 0.05 and I get
> the "right" picture with gnuplot. Is this a known bug? I have not found
> something about it yet.
> 
> I realy need this function, since it is a very important operation in
> complex calculation.
> 
> Thank you!
> 
> 


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