Re: Problem with DSolve
- To: mathgroup at smc.vnet.net
- Subject: [mg96498] Re: Problem with DSolve
- From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
- Date: Sat, 14 Feb 2009 04:14:44 -0500 (EST)
- Organization: The Open University, Milton Keynes, UK
- References: <gn3bnf$pt9$1@smc.vnet.net>
In article <gn3bnf$pt9$1 at smc.vnet.net>, Tony <aezajac at optonline.net>
wrote:
> can anyone help what is wrong?
> On version 7 I enter
>
> DSolve[{y'[x]==.02*y[x]-y[x]^2,y[0]==a},y[x],x]
>
> and get
>
> During evaluation of In[58]:= Solve::ifun: Inverse functions are being
> used by Solve, so some solutions may not be found; use Reduce for
> complete solution information. >>
> During evaluation of In[58]:= Solve::ifun: Inverse functions are being
> used by Solve, so some solutions may not be found; use Reduce for
> complete solution information. >>
> During evaluation of In[58]:= DSolve::bvnul: For some branches of the
> general solution, the given boundary conditions lead to an empty
> solution. >>
As a rule of thumb, one should always use exact numbers whenever
possible when looking for *symbolic* solutions to an equation. Also, it
is a good practice to request the solution as a pure function for it is
easier afterwards to check the validity of the solution. Having done
that, Mathematica 7.0 (and 6.0.3 alike) only returns a particular
solution.
In[1]:= DSolve[{y'[x] == 2/100*y[x] - y[x]^2, y[0] == a}, y, x]
y'[x] == 2/100*y[x] - y[x]^2 /. %[[1]]
{$Version, $ReleaseNumber}
During evaluation of In[1]:= Solve::ifun: Inverse functions are being
used by Solve, so some solutions may not be found; use Reduce for
complete solution information. >>
Out[1]= {{y -> Function[{x}, (a E^(x/50))/(1 - 50 a + 50 a E^(x/50))]}}
Out[2]= True
Out[3]= {"7.0 for Microsoft Windows (32-bit) (November 10, 2008)", 0}
Regards,
--Jean-Marc