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MathGroup Archive 2004

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Re: DSolve change in v5 -- undocumented error message

  • To: mathgroup at smc.vnet.net
  • Subject: [mg46759] Re: DSolve change in v5 -- undocumented error message
  • From: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>
  • Date: Sun, 7 Mar 2004 01:33:37 -0500 (EST)
  • Organization: Universitaet Leipzig
  • References: <c298r9$56c$1@smc.vnet.net>
  • Reply-to: kuska at informatik.uni-leipzig.de
  • Sender: owner-wri-mathgroup at wolfram.com

Hi,

solve it by your self
ss = DSolve[{n'[t] == r*n[t]*(1 - n[t]/k)}, n[t], t]
isol=Solve[(n[t] /. ss[[1]] /. t -> 0) == n0, C[1]];
ss[[1]] /. isol[[1]] // FullSimplify

Regards
  Jens

Gareth Russell wrote:
> 
> Hi,
> 
> The following expression used to work in v4:
> 
> DSolve[{n'[t] == r*n[t]*(1 - n[t]/k), n[0] == n0}, n[t], t]
> 
> In v5, I get the message
> 
> "DSolve::bvfail: For some branches of the general solution, unable to
> solve the conditions."
> 
> Clicking on the "More" link takes me to a page in the Help Browser with
> "Notes for this message have not yet been added."
> 
> Interestingly, the expression without initial conditions works just
> like before
> 
> In[31]:=
> DSolve[{n'[t] == r*n[t]*(1 - n[t]/k)}, n[t], t]
> 
> Out[31]=
> \!\({{n[t] \[Rule] \(\[ExponentialE]\^\(r\ t + k\ C[1]\)\ k\)\/\(\(-1\)
> + \
> \[ExponentialE]\^\(r\ t + k\ C[1]\)\)}}\)
> 
> Does anyone know what is being done differently in v5. And more
> practically, is there a way to recover v4's ability to get a solution.
> Perhaps with assumptions?
> 
> Many thanks,
> 
> Gareth Russell
> Columbia University


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