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MathGroup Archive 2004

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Re: DSolve Question

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50646] Re: DSolve Question
  • From: Kevin <kmccann at umbc.edu>
  • Date: Wed, 15 Sep 2004 01:50:12 -0400 (EDT)
  • Organization: University of Maryland, Baltimore County
  • References: <200409111044.GAA19893@smc.vnet.net> <ci12e0$1e5$1@smc.vnet.net> <ci3ehj$e3f$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

It's not a bug, it is a format problem. This works:

eq1 = {y''[t] + w2 y[t] == 0}
initial = {y[0] == y0, y'[0] == v0}



DSolve[Join[eq1, initial], y[t], t]

Note the brackets on the DE. The format is DSolve[{deqn,ic},var,indvar]

You had DSolve[eq1,{...},var,indvar]

I guess Mathematica didn't like it.

Kevin

John Reed wrote:

> Thanks for bringing up the bug suggestion.  There is documentation about 
> this problem on the Mathematica site at:
> 
> http://support.wolfram.com/mathematica/mathematics/equations/dsolveproblem.html
> 
> I was able to solve the problem using this method.
> 
> John Reed
> 
> "Selwyn Hollis" <sh2.7183 at misspelled.erthlink.net> wrote in message 
> news:ci12e0$1e5$1 at smc.vnet.net...
> 
>>On Sep 11, 2004, at 6:44 AM, John Reed wrote:
>>
>>
>>>I'm relatively new to Mathematica, and I'm having trouble with a
>>>simple DE.
>>>This is from Zimmerman & Olness, "Mathematica for Physics".  Here's the
>>>equation:
>>>
>>>eq1 = y''[t] + w^2 y[t] == 0
>>>
>>>initial = {y[0] == y0, y'[0] == v0}
>>>
>>>eq3 = DSolve[{eq1, initial}, y[t], t]
>>>
>>>When this executes, I get the error message bvnul: For some branches
>>>of the
>>>general solution, the given boundary conditions lead to an empty
>>>solution.
>>>
>>>Now, if I change v0 to a number, everything works fine.  I've tried
>>>making
>>>v0 real and positive, but with no luck.
>>>
>>>Any help is appreciated.
>>>
>>>John Reed
>>
>>
>>Hi John,
>>
>>You've rediscovered a "bug" in DSolve that's new in 5.0. Here's one
>>workaround:
>>
>>  eq1 = y''[t] + w^2 y[t] == 0
>>
>>  initial = {y[0] == y0, y'[0] == v0}
>>
>>  soln[t_] = y[t] /. First@DSolve[eq1, y[t], t]
>>
>>  soln[t] /. First@Solve[initial /. y -> soln, {C[1], C[2]}]
>>
>>
>>
>>-----
>>Selwyn Hollis
>>http://www.appliedsymbols.com
>>(edit reply-to to reply)
>>
> 
> 


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