Re: lsode question

[Juan Pablo Carbajal]

On Wed, Dec 7, 2011 at 12:21 PM, Olaf Till <address@hidden> wrote:
> On Wed, Dec 07, 2011 at 02:15:08AM -0800, dogan_79 wrote:
>> Thank you very much for the nice answers, but I am still uncertain about some
>> things as I am newbie to Octave.
>>
>> My physical problem is to solve the differential equation describing the
>> motion of a point mass inside a spherical bowl which is also subjected to
>> surface friction with the bowl. The friction constant is constant (mu in the
>> code.) What is interesting about this problem is to see the effect of the
>> initial velocity (as the friction will also depend on increasing velocity
>> via centripetal force) So we were discussing the situation with some
>> collegues and none of us had a intuitive feeling about the difference in
>> motion if we threw or released the mass initially.
>
> If lsode gets stuck on this problem I'd take this as a strong advise
> to double-check the differential equations ... and what makes me
> suspicious is the use of the 'sign' function ...
>
> Olaf
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Hi,

If lsode is selecting the discretization automatically (I thought you
force the time step with the time vector as in odepkg) then you can
try controlling some options on the discretization. just type
help lsode
and read carefully

also you can try (assuming you are in GNU/Linux)
pkg -forge install odepkg

and then
help odepkg
(in particular try out first ode45)

However, before meddling with the discrete solver try to understand
your dynamical system. Find the fixed points and their stability (as
Thomas Shores was suggesting). Can you sketch the phase portrait?

If you do not know what I am  talking about, check out these references
http://en.wikipedia.org/wiki/Phase_portrait
http://en.wikipedia.org/wiki/Linear_dynamical_system
http://books.google.com/books/about/Nonlinear_dynamics_and_chaos.html?id=FIYHiBLWCJMC

-- 
M. Sc. Juan Pablo Carbajal
-----
PhD Student
University of Zürich
http://ailab.ifi.uzh.ch/carbajal/