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