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MathGroup Archive 1999

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Re: Plot[] problem (plnr) - help !!!

  • To: mathgroup at smc.vnet.net
  • Subject: [mg15997] Re: [mg15935] Plot[] problem (plnr) - help !!!
  • From: BobHanlon at aol.com
  • Date: Fri, 19 Feb 1999 03:27:10 -0500
  • Sender: owner-wri-mathgroup at wolfram.com

In a message dated 2/18/99 2:21:20 PM, root at borjch.math.tau.ac.il writes:

>I'm trying to do most trivial and simple thing - define 
>some function and then Plot[] it. However, I get some very
>weird error - "  ...is not a machine-size real number at ..."
>
>Here is the code :
>a[0]==1
>a[1]==0.99/50000
>H[w] == 20Log[10,Sum[a[k] Exp[I 2Pi w(-k)], {k,0,1}]]
>Clear[H]
>Plot[H[w], {w, 1, 100}]
>
>I've seen some articles (even at www.wolfram.com) about that
>problem, but NONE of them proposed a working solution !!!
>
>Just putting Evaluate[] inside the Plot[] does nothing...
>

a[0]=1;
a[1]=0.99/50000;
H[w_] := 20Log[10,Sum[a[k] Exp[I 2Pi w(-k)], {k,0,1}]];

Once you clean up the syntax and avoid deleting the definition of H 
before the Plot command, you still have the problem that your 
function is, in general, complex.  For example, 

Table[{w, H[w]}, {w,1,1.5, .1}]//ColumnForm

You can either plot its real part:

Plot[Re[H[w]], {w, 1, 100}];

plot its absolute value:

Plot[Abs[H[w]], {w, 1, 100}];

or, plot its value at integer values of w:

ListPlot[Table[H[w], {w, 1, 100}]];


Bob Hanlon


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