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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: [mg46770] Re: DSolve change in v5 -- undocumented error message
  • From: drbob at bigfoot.com (Bobby R. Treat)
  • Date: Sun, 7 Mar 2004 01:33:49 -0500 (EST)
  • References: <c298r9$56c$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Perhaps this will help you:

DSolve[{n'[t] == r*n[t]*(1 - n[t]/k)}, n[t], t];
n[t] /. First@%
% /. t -> 0
Reduce[n[0] == %, C[1]]
%%% /. Rule @@ First@Cases[%, Equal[C[1], b_]] // FullSimplify

or

DSolve[{n'[t] == r*n[t]*(1 - n[t]/k)}, n[t], t];
n[t] /. First@%
% /. t -> 0
Reduce[n[0] == %, C[1]]
%%% /. Cases[%, a : Equal[C[1], _] :> Rule @@ a] // FullSimplify

Bobby

Gareth Russell <gjr2008 at columbia.edu> wrote in message news:<c298r9$56c$1 at smc.vnet.net>...
> 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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