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