From: <HIDDEN>
Newsgroups: comp.soft-sys.matlab
Subject: Re: ordinary differential equations
Date: Tue, 24 Jan 2012 04:56:10 +0000 (UTC)
Organization: GE Global Research
Lines: 35
Message-ID: <jfldla$nh4$>
References: <jfjpo1$1bp$> <jfkjve$8b4$>
Reply-To: <HIDDEN>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
X-Trace: 1327380970 24100 (24 Jan 2012 04:56:10 GMT)
NNTP-Posting-Date: Tue, 24 Jan 2012 04:56:10 +0000 (UTC)
X-Newsreader: MATLAB Central Newsreader 2872162
Xref: comp.soft-sys.matlab:755578

Thanks Roger, 
No f and g are not inverse of each other but I realize I can reframe my problem
x2_dd=inv_f (x1_dd)

This was confusing me but I guess you are right, ode is probably not appropriate. What about ode15i as Torsten suggested. Can this be treated as implicit ode. 

"Roger Stafford" wrote in message <jfkjve$8b4$>...
> "Pag Max" wrote in message <jfjpo1$1bp$>...
> > I have a set of ode something like
> > 
> > x={x1,x2}
> > 
> > x1_dd=f(x2_dd)
> > and
> > x2_dd=g(x1_dd)
> > .......
> - - - - - - - - - -
>   In the two equations, "x2_dd=g(x1_dd)" and "x1_dd=f(x2_dd)", either 'f' and 'g' are inverses of one another or they are not.  If the first is true, then there is not sufficient information present to solve the differential equations.  You would have in effect only one equation with two unknowns.  A ridiculous example of this would be:
>  dx/dt = 2*dy/dt
>  dy/dt = 1/2*dx/dt
>  x(0) = 3
>  y(0) = -2
> Now solve for x(t) and y(t).  As you can easily see, there is a vast infinitude of possible solutions since dy/dt could be any function of t whatever, as long as dx/dt were twice that.
>   On the other hand if 'f' and 'g' are not inverses of one another, there is presumably only one or at least only a finite set of constant number pairs that are possible values for x1_dd and x2_dd, in which case you are dealing with a trivial problem of linear functions of the independent variable where the derivatives are constants.
>   In neither of the above situations is the use of 'ode' functions appropriate.
> Roger Stafford