       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==a},y[x],x]
>
> and get
>
>        During evaluation of In:= 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:= 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:= 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:= DSolve[{y'[x] == 2/100*y[x] - y[x]^2, y == a}, y, x]
y'[x] == 2/100*y[x] - y[x]^2 /. %[]
{\$Version, \$ReleaseNumber}

During evaluation of In:= Solve::ifun: Inverse functions are being
used by Solve, so some solutions may not be found; use Reduce for
complete solution information. >>

Out= {{y -> Function[{x}, (a E^(x/50))/(1 - 50 a + 50 a E^(x/50))]}}

Out= True

Out= {"7.0 for Microsoft Windows (32-bit) (November 10, 2008)", 0}

Regards,
--Jean-Marc

```

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