Re: Help need for Differential algebraic equations

[Carlo de Falco]

On 09/ott/08, at 18:09, genehacker wrote:


> **********code ********
> function res = f(x,xdot,t);
> res = zeros(3,1);
> k12=0.1;
> k23=0.7;
>
> xdot(1) = -k12*x(1);
> xdot(2) = k12*x(1) - k23*x(2);
> xdot(3) = k23*x(2);

> endfunction;

as it stands this function does not make much sense:
whatever its input is it will always return [0 0 0].'

to check, try for example running the following command multiple times:
f (rand(3,1), rand(3,1), rand(1))

as xdot is not an output of the function, whatever value
you assign to it will be lost when the function returns.

I suggest you read the manual chapter on defining functions:
http://www.gnu.org/software/octave/doc/interpreter/Functions-and-Scripts.html#Functions-and-Scripts
to understand how input and output parameters are handled

> x0 = [100,0,0];
> xdot0 =[-10,10,10];
> t = linspace(0,50,50);
> [x,xdot] = dassl("f",x0,xdot0,t);


the first sentence in the documentation of dassl reads:

"-- Loadable Function: [X, XDOT, ISTATE, MSG] = dassl (FCN, X_0,  
XDOT_0, T, T_CRIT)
Solve the set of differential-algebraic equations
          0 = f (x, xdot, t)"

so, put in mathematical terms, the problem you are trying to solve  
with this statement reads:

find x(t) such that f(x,x',t) = 0 for 0<= t <= 50

as with your definition  f(x,x',t) = 0 is verified for any value of x,  
x' and t,
the problem is not well posed as ANY function x(t) is a solution.

c.