[SciPy-user] Problems with odeint

[Vinicius Lobosco]

Hi Victor!

I just ran your code on my machine to take a closer look and I goot some
nice results, which at first glance seem to be right. Please, check the
attached figure.

/Vinicius

On 6/14/06, Víctor Martínez-Moll <victor.martinez at uib.es> wrote:
>
> Hi again,
> Sorry for bothering again but as my message arrived just before the list
> problem and I've received no answer I'm wondering if maybe some message
> was lost.
>
> So, again, any suggestions on how to solve this simple 2nd order
> differential equation with Scipy? Or, what the hell I did wrong to get
> such strangre results?
>
> Thanks in advance and best regards.
>
> Victor
>
> PS: If you think this is not the right forum or you think I could find
> the answer to my question reading any particular documentation please
> let me know too. I've read some of the documentation I found online but
> maybe I've missed something.
>
> En/na Víctor Martínez-Moll ha escrit:
> > 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:
> >
> > ######################################
> > ## The Python one                   ##
> > ######################################
> >
> > from scipy import *
> > from matplotlib.pylab import *
> >
> > def dwdt(w,t):
> >      return [w[1],1.0-sin(w[0])]
> >
> > t=arange(0.0,2.0*pi,.01)
> >
> > ww = integrate.odeint(dwdt,[0.0,0.0],t,hmin=0.01,atol=.001)
> >
> > y = ww[:,0]
> > dy =ww[:,1]
> > ddy = 1.0-sin(ww[:,0])
> >
> > plot(t,y,label='y')
> > plot(t,dy,label='dy')
> > plot(t,ddy,label='ddy')
> > legend()
> > show()
> >
> > ######################################
> > ## The SciLab one                   ##
> > ######################################
> >
> > function result = dwdt(t,w)
> >    result(1) = w(2)
> >    result(2) = 1.0-sin(w(1))
> > endfunction
> >
> > t = 0.0:0.01:2.0*%pi;
> >
> > ww = ode([0.0;0.0],0.0,t,dwdt);
> >
> > y = ww(1,:);
> > dy = ww(2,:);
> > ddy=1.0-sin(ww(1,:));
> >
> > xset("window", 0)
> > xbasc()
> > plot2d(t,[y;dy;ddy]',leg="y at dy@ddy")
> >
> > ########################################
> >
> > Any ideas or suggestions will be wellcome.
> >
> >
>
> --
> Víctor Martínez Moll           | Universitat de les Illes Balears
> Departament de Física          | Edifici Mateu Orfila
> Àrea d'Enginyeria Mecànica     | E-07122, Palma de Mallorca, SPAIN
> e-mail: victor.martinez at uib.es | Tel:34-971171374 Fax:34-971173426
>
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> SciPy-user at scipy.net
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>



-- 
---------------------------------
Vinicius Lobosco, PhD
www.paperplat.com

+46   8 612 7803
+46 73 925 8476

Björnnäsvägen 21
SE-113 47 Stockholm, Sweden
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