       Difficulty with NDSolve (and DSolve)

• To: mathgroup at smc.vnet.net
• Subject: [mg106125] Difficulty with NDSolve (and DSolve)
• From: KK <kknatarajan at yahoo.com>
• Date: Sat, 2 Jan 2010 05:04:58 -0500 (EST)

```Hi,

I am trying to numerically solve a differential equation. However, I
am encountering difficulty with Mathematica in generating valid
numerical solutions even for a special case of that equation.

The differential equation for the special case is:
F'[x] == - (2-F[x])^2/(1-2 x + x F[x]) and
F==1.
These equations are defined for x in (0,1).  Moreover, for my context,
I am only interested in solutions with F[x] in the range <1.

Even before I used Mathematica, I had computed the solution to the
differential equation as the solution to the following : x (2-F[x]) -
Log[2-F[x]]==1.  For any given x in (0,1), there are two values of F
[x] that satifsy the equation.  One of them is always less than 1, and
the other is F[x]= 2 + (ProductLog[E^(I (I + \[Pi])) x])/x which is
always greater than 1.  For example, when x=0.85, the solutions are F
[x]=0.2979 and F[x]=1.32407.  As mentioned earlier, I am only
interested in the first solution.

Both DSolve and NDSolve seem to provide only the second solution and
not the first one.  When using DSolve, it gives out a warning that
there may be multiple solutions but that's about it.  Is there an
option or something that I can set with NDSolve (or even DSolve) to
generate the solutions of interest to me?

Below is the relevant set of Mathematica code and the corresponding
outputs.  I would greatly appreciate your help in this regard.

Thanks,
KK

------------------
In:= DSolve[{ -((-2 + F[x])^2/(1 - 2 x + x F[x])) == F'[x],
F == 1}, F[x], x]

During evaluation of In:= InverseFunction::ifun: Inverse functions
are being used. Values may be lost for multivalued inverses. >>

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. >>

Out= {{F[x] -> (2 x + ProductLog[E^(I (I + \[Pi])) x])/x}}
------------------

(*Sample solution*)
In:= Solve[(1 == (x (2 - F[x]) - Log[2 - F[x]] /. F[x] -> F) /.
x -> 0.9), F]

During evaluation of In:= InverseFunction::ifun: Inverse functions
are being used. Values may be lost for multivalued inverses. >>

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= {{F -> 0.297987}, {F -> 1.32407}}

--------------------
(*Please note I am using F[0.9999]=1 as the initial condition in the
NDSolve below.  \
Otherwise, I end up with a warning that "Infinite expression 1/0. \
encountered. I had also used the same initial condition with DSolve \
and the results were similar.*)

In:= NumSol =
NDSolve[{ -((-2 + F[x])^2/(1 - 2 x + x F[x])) == F'[x], F[.9999] ==
1}, F, {x, 0, 1}]

During evaluation of In:= NDSolve::ndsz: At x ==
0.9999000049911249`, step size is effectively zero; singularity or
stiff system suspected. >>

Out= {{F -> \!\(\*
TagBox[
RowBox[{"InterpolatingFunction", "[",
RowBox[{
RowBox[{"{",
RowBox[{"{",
RowBox[{"0.`", ",", "0.9999000049911249`"}], "}"}], "}"}],
",", "\<\"<>\"\>"}], "]"}],
False,
Editable->False]\)}}
---
(* I have omitted the plot here but it generates only the second
result*)
In:= Plot[{Evaluate[F[x] /. NumSol]}, {x, .33, 1}]
-----------------------

```

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