Re: octave and delay ode's

[Carlo de Falco]

On 1 Nov 2009, at 20:08, franco basaglia wrote:

> I must be missing something as in your file I see no time  
> derivatives...
> sorry for my oversight. I attach again pdf with right system.

What exactly does "delta t" mean in your file?

> function f = fun (t, y, yd)
> f(1) =-y(1) + yd(2);
> f(2) =-y(2) ;
> endfunction
>
> ode23d (@fun, [0 5], [1;1], .1, [1;1])
>
> notice that in the example above only one equation includes a delay:
> is this what you mean by system of ODEs and DDEs?
>
> Yes,sure.
> To be more precise I have a system like this:
> d y1(t)/ dt = -y1(t)
> d y2(t)/ dt = -y2(t) + y1(t-5)
> d y3(t)/dt  = -y3(t) + y2(t-10)*y1(t-10)
>
> that I try to solve in this way:
> function f = fun (t, y, yd)
> f(1) =-y(1)
> f(2) =-y(2) + yd(1)
> f(3) =-y(3) + yd(2)*yd(1)
> endfunction
> t = [0:.5:20]
> res = ode45d (@fun, t, [1;1;1], [5;10], ones (3,10))

I think this should rather be something like:

function f = fun (t, y, yd)
f(1) =-y(1);                   %% y1' = -y1(t)
f(2) =-y(2) + yd(1,1);         %% y2' = -y2(t) + y1(t-lags(1))
f(3) =-y(3) + yd(2,2)*yd(1,2); %% y3' = -y3(t) + y2(t-lags(2))*y1(t- 
lags(2))
endfunction

T = [0,20]                                            %% only initlial  
and final time
                                                      %% need to be  
provided

res = ode45d (@fun, t, [1;1;1], [5, 10], ones (3,2)); %% number of  
columns in HIST should
                                                      %% be the same  
as the number of columns
                                                      %% in LAGS

I beleive Thomas Treichl, the author of odepkg is listening on the  
list so maybe he can confirm whether this is correct?

> It's run. But I don't think this is the right way.I don't know if  
> with this notation it applies a delay of 5 time units to f(2) and of  
> 10 time units to f(3).
what do you mean by "time units"?

> Note that in f(3) yd(1) comes again but with a different delay.
> Infact for my script, that I attach here, is the same. It's runs but  
> with different results compare to Stella.
>
> Any Idea?

From your pdf file and your script I get the feeling that you are  
confusing the
time parameter in the continuos problem and the number of time-steps  
in the discretized
system...

> thanks a lot
>
> f.t.
c.

P.S. please keep the list in CC as someone else may want to help with  
your questions
or benefit from the answers.

equazioniPatu.pdf
Description: Adobe PDF document

scriptode45.m
Description: Text Data