Re: 2 coupled diff. eqns

• To: mathgroup at smc.vnet.net
• Subject: [mg21178] Re: 2 coupled diff. eqns
• From: "Kevin J. McCann" <kevin.mccann at jhuapl.edu>
• Date: Fri, 17 Dec 1999 01:23:09 -0500 (EST)
• Organization: Johns Hopkins University Applied Physics Lab, Laurel, MD, USA
• References: <831vap\$g58@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```The "\)" in your DSolve call should not be there, but, unfortunately, DSolve
still doesn't solve it, but takes a long time to figure it out (~3 min).

Kevin

--

Kevin J. McCann
Johns Hopkins University APL

Henk Jansen <hj at rdr-nl.com> wrote in message news:831vap\$g58 at smc.vnet.net...
> I have the following set of two coupled differential equations:
>
>                     c f
>    f' = a -  ----------------
>                  ___________
>              \  /  2    2
>               \/  f  + g
>
>                     c g
>    g' = b -  ----------------
>                  ___________
>              \  /  2    2
>               \/  f  + g
>
>
> where a, b and c are constants. Trying to solve this system (if
> possible), after typing
>
> DSolve[
>     {D[f[t], t] == a - (w f[t])/(Sqrt[f[t]^2 + g[t]^2])\),
>      D[g[t], t] == b - (w g[t])/(Sqrt[f[t]^2 + g[t]^2])\)
>     },
>     {f[t], g[t]},
>     t]
>
> Mathematica returns with the following message:
>
>    "Part::partw: Part 2 of g'[f] does not exist."
>
> without solution. I have two questions:
>
> 1. Does anyone know how to interprete this message?
>
> 2. If the system is not solvable, is there a clever coordinate
> transformation for which the system can be solved?
>
> Thanks,
>
> Henk Jansen
>
> --
> = = Henk Jansen ======== H.Jansen at fel.tno.nl = =
> = == TNO Physics and Electronics Laboratory == =
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