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MathGroup Archive 1996

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differential equations

  • To: mathgroup at smc.vnet.net
  • Subject: [mg4522] differential equations
  • From: N Kinoshita <nk204 at cam.ac.uk>
  • Date: Wed, 7 Aug 1996 04:17:32 -0400
  • Sender: owner-wri-mathgroup at wolfram.com

Hi, I am a research student of economics. I want to solve
a system of differential equations with Mathematica.

I wrote the follwoing command based on the example given
in p.698 of 'Mathematica: A system for doing mathematics
by computer'
 
------------------------------------------------------------
ks=0.380884;cs=0.0282254;ws=0.846308;
k0=ks/10;
cprimess=0.00778013/0.257126;
wprimess=0.966347/0.257126;

NDSolve[
{c'[k]==
 If[k==ks,cprimess,
    (-0.012+0.952757k/w[k]^.7+0.11938/w[k]^.6
    +2.6603c[k]/w[k]^.1-0.635171w[k])
    /(0.875c[k]+0.952757k^2/w[k]^.7+1.04457k/w[k]^.6
    +2.6603c[k]k/w[k]^.1-0.635171k*w[k]^.3-0.447674w[k]^.4)
    ],
 w'[k]==
 If[k==ks,wprimess,
    (1.1938w[k]^.4+6.65074c[k]w[k]^.9-1.58793w[k]^1.3)
    /(0.875c[k]+0.952757k^2/w[k]^.7+1.04457k/w[k]^.6
    +2.6603c[k]k/w[k]^.1-0.635171k*w[k]^.3-0.447674w[k]^.4)
    ],
 c[ks]==cs, w[ks]==ws},
{c,w},{k,ks,k0}]
-----------------------------------------------------------

Then I got the error message saying:
   NDSolve::deql:
     True is not a valid equation or list of valid equations.

I was able to solve a single differential equation before,
but do not know how to cope with my problem now.

I would like to get some comments.



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