       Re: Problem with DSolve

• To: mathgroup at smc.vnet.net
• Subject: [mg96476] Re: [mg96410] Problem with DSolve
• From: Daniel Lichtblau <danl at wolfram.com>
• Date: Sat, 14 Feb 2009 03:10:54 -0500 (EST)
• References: <200902130841.DAA26548@smc.vnet.net>

```Tony 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. >>

You can see all calls to Solve, as below.

In:= Solve[a___] := Null /; (Print[InputForm[solve[a]]]; False)

In:= DSolve[{y'[x]==.02*y[x]-y[x]^2,y==a},y[x],x]
solve[-0.02*y[x] + 1.*y[x]^2 + 1.*Derivative[y][x] == 0,

>   Derivative[y][x]]
solve[-Log[y[x]] + Log[-1 + 50*y[x]] == -x/50 + C, y[x]]
solve[{(1. - 50.*a + 1.*2.718281828459045^C*a)/

>      (50. - 1.*2.718281828459045^C) == 0}, {C},

>   VerifySolutions -> True]

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

0.02 x
2.71828
Out= {{y[x] -> -------------------------------------------}}
0.02 x       -1. (1. - 50. a) 1.
50. 2.71828       - 1. (----------------)
a

The one that gives rise to the message involves solving for the
constant, and it appears in exponents. Hence the warning message.

Daniel Lichtblau
Wolfram Research

```

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