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MathGroup Archive 2001

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Re: desperate, please help.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27362] Re: desperate, please help.
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Thu, 22 Feb 2001 02:25:07 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <97003k$tq@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

Mathematica can't solve differential algebraic equations.
More over that is a bit difficult because only
for special input values a solution exist.

Don't type stuff like ÷ into equations, don't make 
assignments like F = .9*163 in differential equations,
don't mix the case in k[x] and K[x], use always an argument
(not a single K or r) and try to elimimate T[x],F,C,K[x]..
with Eliminate[] before you call NDSolve[].

Regards
  Jens

Thjqase wrote:
> 
> I need help numerically solving a differential equation coupled with several
> algebraic equations. New to Mathematica. Here's code:
> 
> NDSolve[{v'[x] == -F÷r, T[x] == 330 + 43.3*x, F = .9*163, C == 9.3,
>     k[x] == 31.1Exp(7906*(T - 360)/(T*360))
>     , K[x] == 3.03*Exp(-830.3*((T - 333)/(T*333))),
>     r[x] == -k*C*(1 - (1 + 1/K)*x),
>     v[0] == 0}, {v}
>   , {x, 0, .7}]
> 
> Error code is:
> 
> NDSolve::"deql": "The first argument must have both an equation and an \
> initial condition."
> 
> Any suggestions? What am I doing wrong? My head is sore from banging it against
> the wall.
> 
> TIA.


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