[SciPy-user] Problems with odeint

[Fernando Perez]

On 6/9/06, Víctor Martínez-Moll <victor.martinez at uib.es> wrote:
> Hi all,
>
> I've been a SciLab user for some time and I'm evaluating SciPy as a
> development tool.
>
> The first thing I tried is to solve a simple second order diferential
> equation using odeint(). The problem is that depending on the function I
> want to integrate I get nice results, but for most of them I get simply
> nothing or nonsense answers. Is not a problem of the function having a
> strange behaviour or having singularity points. For example if I try to
> solve:
>      d2y/dt2 = 1-sin(y)
> either I get nothing or wrong solutions (the best thing I got was
> setting:hmin=0.01,atol=.001), while If I do about the same procedure in
> SciLab I get a nice and smooth set of curves. The strangest thing is
> that if I use exactly the same procedure to solve:
>      d2y/dt2 = 1-y
> then I get the right solution, which seems to indicate that I'm doing
> the right thing (although of course I know I'm not because I do not
> belive that odeint is not able to solve such a silly thing).
>
> I've only checked it with the last enthon distribution I found:
> enthon-python2.4-1.0.0.beta2.exe
>
> The simple procedure I wrote in Python and its equivalent in SciLab that
> does the right thing in are:

I'm sorry, but I get basically the same results with both.  I'm
attaching a slightly modified version of the python script you wrote,
which prints some debug information.  On my system, this information
reads:

In [7]: run odebug
numerix flag : Numeric
mpl   version: 0.87.3
scipy version: 0.5.0.1940

I've attached a png of the resulting plot.

I've never used scilab before, but just brute-pasting your example
into a scilab window and exporting the resulting plot gave me the
attached file.


>From what I can see, both results look more or less consistent (I
haven't done numerical accuracy checks, I'm just looking at the
figures).

Cheers,

f
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