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MathGroup Archive 1999

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Re: Some problems with DSolve and NDSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg19775] Re: Some problems with DSolve and NDSolve
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Fri, 17 Sep 1999 01:36:36 -0400
  • Organization: Universitaet Leipzig
  • References: <7rnjb1$ejs@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

 the system is singular, Mathematica can't find two explicit
 differential equations for i1'[t] und i2'[t]. Your equations say
 that deltaI'[t]=i2'[t]-i1'[t]
 is given by
  

  2 i1[t] - 2 deltaI'[t] == 80 t E^(-2t), 4 i2[t] + 2 deltaI'[t] == 0

Hope that helps
  Jens 



Chee Lim Cheung wrote:
> 
> Hi MathGroupers,
> 
> I was somewhat surprised that DSolve is unable to solve the following
> relatively innocuous system of 1st-order ODEs:
> 
> DSolve[{2 i1[t] + 2 (i1'[t] - i2'[t]) == 80 t E^(-2t), 4 i2[t] + 2 (i2'[t]
> - i1'[t]) == 0, i1[0] == 0, i2[0] == 0}, {i1[t],i2[t]}, t]
> 
> The normally reliable NDSolve also falls down on the following:
> 
> NDSolve[{2 i1[t] + 2(i1'[t] - i2'[t]) == 80 t E^(-2t), 4 i2[t] + 2(i2'[t] -
> i1'[t]) == 0, i1[0] == 0, i2[0] == 0}, {i1[t], i2[t]}, {t, 0, 1}]
> 
> What is wrong?
> 
> Thanks
> Chee


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