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MathGroup Archive 2004

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DSolve question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49129] DSolve question
  • From: "James A. Smith" <jasmith at cim.mcgill.ca>
  • Date: Fri, 2 Jul 2004 02:01:33 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

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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