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MathGroup Archive 2004

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Re: DSolve question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49138] Re: DSolve question
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Mon, 5 Jul 2004 04:54:17 -0400 (EDT)
  • Organization: Universitaet Leipzig
  • References: <cc2uk2$5i1$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

you must have somwhere in your notebook an assignment to x.

Regards
  Jens

"James A. Smith" wrote:
> 
> Being a Mathematica novice, I'm confused by an error generated when I
> modify the following differential equation example:
> 
> Input 1: DSolve[y'[x] == 2 a x, y[x], x]
> Output 1: {{y[x] -> a x^2 + C[1]}}
> 
> This is fine.  But why does the following produce an error, since all I am
> doing is replacing the independent "x" variable with "t" and the
> dependent "y" variable with "x":
> 
> Input 2: DSolve[x'[t] == 2 a t, x[t], t]
> (error) DSolve::dvnoarg: The function x appears with no arguments.
> 
> As far as I can tell, I've only done a simple find-and-replace of the
> variable names.
> 
> I looked up a related "dvnoarg" problem that appeared on the MathGroup
> archive (http://forums.wolfram.com/mathgroup/archive/2003/Dec/msg00119.html)
> and performed the same fine-and-replace:
> 
> Input 3: DSolve[y''[x] == ay'[x] + y[x], y[x], x]
> Input 4: DSolve[x''[t] == ax'[t] + x[t], x[t], t]
> 
> Both of these seem to work out fine.
> 
> I'm using Mathematica 4.2 (OS X)...
> 
> Thank you,
> 
> James Smith


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