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MathGroup Archive 2006

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Re: Plot resolution

  • To: mathgroup at smc.vnet.net
  • Subject: [mg65098] Re: Plot resolution
  • From: "ben" <benjamin.friedrich at gmail.com>
  • Date: Tue, 14 Mar 2006 06:00:26 -0500 (EST)
  • References: <dv31e3$r6d$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi Laurentiu,

Your problem is about precision:
Subtracting huge numbers yielding tiny differences is always subtle.

A quick'n'dirty work-around is

ListPlot[Table[N[pressure, 20], {r, ro, 10^4, 10^2}],PlotJoined ->
True]

Here N[ ,20] simply uses more digits in the calculation.

Bye
Ben

Laurentiu Caramete schrieb:

> Hi,
>
> I got a problem with a plot of a function. The function 'pressure' should
> decrease monotonically with r. The Plot function is giving a non-monotonic
> plot at big r, this is a problem with the resolution of the plot or with the
> function? How can I check that?
>
> \!\(Clear[r]\[IndentingNewLine]
>   \(ro = 10\^3;\)\[IndentingNewLine]
>   \(pressure =
>       p[r] /. \(DSolve[{D[p[r], r] ==
>                 1\/\(\(r\^2\) \((1 + r)\)\^3\) -
>                   Log[1 + r]\/\(\(r\^3\) \((1 + r)\)\^2\),
>               p[ro] == 10\^\(-7\)}, p[r],
>             r]\)[\([1]\)];\)\[IndentingNewLine]\[IndentingNewLine]
>   Plot[pressure, {r, ro, 10\^4}, PlotRange -> All]\)
> 
> 
> Thanks


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