       Re: Getting only 1 of 3 curves of intersection

• To: mathgroup at smc.vnet.net
• Subject: [mg126315] Re: Getting only 1 of 3 curves of intersection
• From: "djmpark" <djmpark at comcast.net>
• Date: Mon, 30 Apr 2012 04:43:43 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <1198356.198995.1335681120499.JavaMail.root@m06>

```Bill,

Use Reduce:

Reduce[Cos[x y] == Sin[x y] && 0 <= y <= \[Pi], {x, y}, Reals]

Then write ConditionalExpressions for the intersections.

intersection1[k_][x_] =
ConditionalExpression[{x, (-2 ArcTan[1 + Sqrt] + 2 \[Pi] k)/x,
Sin[-2 ArcTan[1 + Sqrt] + 2 \[Pi] k]}, (k <= 0 &&
x <= (-2 ArcTan[1 + Sqrt] + 2 \[Pi] k)/\[Pi]) || (k >= 1 &&
x >= (-2 ArcTan[1 + Sqrt] + 2 \[Pi] k)/\[Pi])];

intersection2[k_][x_] =
ConditionalExpression[{x, (-2 ArcTan[1 - Sqrt] + 2 \[Pi] k)/x,
Sin[-2 ArcTan[1 - Sqrt] + 2 \[Pi] k]}, (k <= -1 &&
x <= (-2 ArcTan[1 - Sqrt] + 2 \[Pi] k)/\[Pi]) || (k >= 0 &&
x >= (-2 ArcTan[1 - Sqrt] + 2 \[Pi] k)/\[Pi])];

The particular intersections you want for your plot are:

intersection2[x]
intersection2[x]
intersection1[x]

In Presentations I would draw this as:

<< Presentations`

Draw3DItems[
{{Opacity[0.6, ColorData["Crayola"]["RedViolet"]],
Draw3D[Sin[x y], {x, 0, Pi}, {y, 0, Pi}, Mesh -> None]},
{Opacity[0.6, Cyan],
Draw3D[Cos[x y], {x, 0, Pi}, {y, 0, Pi}, Mesh -> None]},
{Black, Thick,
ParametricDraw3D[intersection2[x], {x, 0, \[Pi]}],
ParametricDraw3D[intersection2[x], {x, 0, \[Pi]}],
ParametricDraw3D[intersection1[x], {x, 0, \[Pi]}]}},
NeutralLighting[0, 0.5, 0.4],
NiceRotation,
Axes -> True, AxesLabel -> {"x", "y", "z"},
BoxRatios -> {1, 1, 0.5},
ImageSize -> 500]

David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/index.html

From: Bill [mailto:WDWNORWALK at aol.com]

Hi:

Consider the following plot:

g1=Plot3D[Sin[x*y],{x,0,Pi},{y,0,Pi},PlotStyle->None,MeshStyle->Red,Axes->Tr
ue,AxesLabel->{"x","y","z"}];
g2=Plot3D[Cos[x*y],{x,0,Pi},{y,0,Pi},Mesh->None,PlotStyle->{Cyan,Opacity[.8]
}];
nsSol=NSolve[{z-Sin[x*y],z-Cos[x*y]},{y,z}];//Quiet
g3=ParametricPlot3D[{x,y,z}/.nsSol[],{x,0,Pi},PlotStyle->{Magenta,Thickne
ss[.007]}];
Show[g1,g2,g3,Background->LightYellow,ImageSize->500]

Using the above Mathematica 8.0.4 code, I can plot one curve of intersection
shown in magenta.
Within the plotted area, I can see 2 more places where intersection curves
should be.
I've tried using Reduce in place of NSolve, but can't get it to work.

Question: If this can be done, can someone please give me the code?

Thanks,

Bill

```

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