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Re: DSolve change in v5 -- undocumented error message
*To*: mathgroup at smc.vnet.net
*Subject*: [mg46762] Re: DSolve change in v5 -- undocumented error message
*From*: bobhanlon at aol.com (Bob Hanlon)
*Date*: Sun, 7 Mar 2004 01:33:40 -0500 (EST)
*References*: <c298r9$56c$1@smc.vnet.net>
*Sender*: owner-wri-mathgroup at wolfram.com
eqn=n'[t]==r*n[t]*(1-n[t]/k);
ans1 =DSolve[eqn,n[t],t][[1]];
Off[Solve::ifun];
ans2 = ans1 /.
Solve[(n[0] /.
(ans1 /. t->0))==n0,
C[1]][[1]] // Simplify
{n[t] -> (E^(r*t)*k*n0)/
(k + (-1 + E^(r*t))*n0)}
ans2 /. t->0
{n[0] -> n0}
eqn /.Join[ans2,
D[ans2, t]//Simplify]//
Simplify
True
Bob Hanlon
In article <c298r9$56c$1 at smc.vnet.net>, Gareth Russell <gjr2008 at columbia.edu>
wrote:
<< 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?
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