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)
• 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
>
> I realy need this function, since it is a very important operation in
> complex calculation.
>
> Thank you!
>
>

```

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