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MathGroup Archive 2002

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Re: Why these graphs differ?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg33943] Re: [mg33925] Why these graphs differ?
  • From: Andrzej Kozlowski <andrzej at platon.c.u-tokyo.ac.jp>
  • Date: Wed, 24 Apr 2002 01:21:52 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

Really it is neither. What it really means is that you should study 
Mathematica a little more. The second Plot is correct while the first is 
basically nonsense. What you did is make Mathematica try to solve a lot 
of "differential equations" like this one:

DSolve[{Derivative[1][y][4.166666666666666*^-8] ==
       4.166666666666666*^-8, y[0] == 1},
     y[4.166666666666666*^-8], 4.166666666666666*^-8][[1,1, 2]]

As you can see yourself this does not make much sense. It is not a bug, 
just a very basic fact about the way Plot evaluates its arguments.

Andrzej Kozlowski
Toyama International University
JAPAN
http://platon.c.u-tokyo.ac.jp/andrzej/


On Tuesday, April 23, 2002, at 08:13  PM, Vladimir Bondarenko wrote:

> These solutions, naturally, coincide.
>
>          DSolve[{y'[z] == z, y[0] == 1}, y[z], z][[1, 1, 2]]
> Evaluate[DSolve[{y'[z] == z, y[0] == 1}, y[z], z][[1, 1, 2]]]
>
> (2 + z^2)/2
> (2 + z^2)/2
>
>
> But, surprisingly, the corresponding graphs are not identical:
>
> Plot[         DSolve[{y'[z] == z, y[0] == 1}, y[z], z][[1, 1, 2]],  {z, 
> 0, 1}]
> Plot[Evaluate[DSolve[{y'[z] == z, y[0] == 1}, y[z], z][[1, 1, 2]]], {z, 
> 0, 1}]
>
>
> Is it a feature or a problem?
>
>
> Vladimir Bondarenko
>
>
>
>
>



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