Re: DSolve Question

• To: mathgroup at smc.vnet.net
• Subject: [mg50615] Re: DSolve Question
• From: "John Reed" <nospamjreed at alum.mit.edu>
• Date: Mon, 13 Sep 2004 02:19:30 -0400 (EDT)
• References: <200409111044.GAA19893@smc.vnet.net> <ci12e0\$1e5\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```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