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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