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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[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. >>

You can see all calls to Solve, as below.

In[12]:= Solve[a___] := Null /; (Print[InputForm[solve[a]]]; False)

In[13]:= DSolve[{y'[x]==.02*y[x]-y[x]^2,y[0]==a},y[x],x]
solve[-0.02*y[x] + 1.*y[x]^2 + 1.*Derivative[1][y][x] == 0,

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

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

 >   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[13]= {{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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