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Re: Can someone tell me why NDsolve isn't working here?

• To: mathgroup at smc.vnet.net
• Subject: [mg44040] Re: Can someone tell me why NDsolve isn't working here?
• From: "Eckhard Hennig" <aidev at n-o-s-p-a-m.kaninkolo.de>
• Date: Sun, 19 Oct 2003 01:10:34 -0400 (EDT)
• References: <bmqplo$rhb$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

"Mizrandir" <mizrandir at hotmail.com> schrieb im Newsbeitrag
news:bmqplo$rhb$1 at smc.vnet.net...
> Hello, Im trying to solve a system of 6 differential equations with
> NDSolve but I can't get Mathematica to do it. I wrote:
[...]
> And Mathematica gave me the following error:
>
> NDSolve::"ndode": "Input is not an ordinary differential equation."

You're getting this error because your equations are of DAE type
(differential-algebraic equations) rather than ODE. Unlike ODEs, DAEs
involve algebraic (i.e. non-differential) constraints, such as Ry[t] + Fy[t]
== 0. NDSolve cannot solve this type of equations, at least for Mathematica
versions < 5.

You can try to solve your equations with the NDAESolve command from Analog
Insydes (www.analog-insydes.de), which has been designed for solving
electrical circuit equations. Its trapezoidal integrator may not be ideal
for your application though (looks to me like a mechanical system where
energy conservation is important).

Best regards,

  Eckhard

--
Dr.-Ing. Eckhard Hennig
www.kaninkolo.de/ai
aidev \at kaninkolo \dot de