       Need code to calculate the Lower Envelope for a set of (non collinear) points.

• To: mathgroup at smc.vnet.net
• Subject: [mg51409] Need code to calculate the Lower Envelope for a set of (non collinear) points.
• From: gilmar.rodriguez at nwfwmd.state.fl.us (Gilmar Rodr?guez Pierluissi)
• Date: Sat, 16 Oct 2004 04:20:45 -0400 (EDT)
• Sender: owner-wri-mathgroup at wolfram.com

```Dear Mathematica Solver Group:

I'm looking for a program to calculate the Lower Envelope
("LE" for short)for a set of (non-collinear) points on the
arbitrary set of such points(which I'm calling "A") by
double-clicking the following shortcut:

to evaluate the following steps using Mathematica (version 5):

evaluate the following Mathematica commands:

In: A = ReadList["C:\\Temporary\\A.txt", {Number, Number}];

Next, plot the set A:

In: plt1 = ListPlot[A, PlotJoined -> True, PlotStyle -> {Hue[.7]}]

I can manipulate the program ConvexHull to find the Lower Envelope
for the set A as follows:

In: << DiscreteMath`ComputationalGeometry`

In: convexhull = ConvexHull[A]

The following input gives a picture of the Convex Hull:

In: plt2 = ListPlot[Table[A[[convexhull[[i]]]], {i,
1, Length[convexhull]}], PlotJoined -> True, PlotStyle -> {Hue[.6]}]

Modifying the starting value of index i in In above
(starting at i=96 instead of i=1) gives a picture of the
Lower Envelope of A:

In:plt3 = ListPlot[Table[A[[convexhull[[i]]]], {i,
96, Length[convexhull]}], PlotJoined -> True, PlotStyle -> {Hue[.6]}]

In: Show[plt1, plt3]

The Lower Envelope of A ("LEA" for short) is given by:

In: LEA = Table[A[[convexhull[[i]]]], {i, 96, Length[convexhull]}]

So my question is: How can the code of the ConvexHull program be modified,
to get a program that calculates the LE of a set?

The following (clumsy)alternative attempt:

In: LE[B_] := Module[{M}, {L = Length[B]; M = {};
AppendTo[M, B[[L]]]; {Xg, Yg} = B[[L]]; Do[If[B[[
L - i + 1]][] < Yg, {{Xg, Yg} = B[[L -
i + 1]], AppendTo[M, B[[L - i + 1]]]}], {i, 1, L - 1}]}; Sort[M]]

In: LE[A]

In: plt4 = ListPlot[LE[A], PlotJoined -> True, PlotStyle -> {Hue[.1]}]

In: Show[plt1, plt4]

gives me something that is "not even close, and no cigar".

What I need is an algorithm that gives me the Lower Envelope E of A
as shown in In above.  Thank you for your help!

```

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