Re: function writing in differential equations

[Olaf Till]

On Mon, Oct 13, 2008 at 04:06:26PM -0700, genehacker wrote:
> 
> Hi,
> 
>      I am using dassl to solve algebraic differential equations and want to
> write a variable as a function of time according to the dassl condition 0 =
> f(x,xdot,t)
> 
> If I tried writing x(1) as a function only on time exponentially, like this,
> x(1) = exp(-10*t), it gives an error. I would like to know the correct way
> of writing a variable dependent only on time, if the function that solves
> the ADE is function res = f(x,xdot,t);
> Please help.

A variable which only depends on time does not belong into the vector
'x'. Cancel the equation for 'x(1)' from your function 'f'. If some
other equation in 'f' needed 'x(1)', substitute 'exp(-10*t)'
instead. If some other equation in 'f' needed 'xdot(1)', substitute
the derivative of 'x(1)' (-10*exp(-10*t)) instead.

Any variable which does not depend only on time, but its derivative
plays no role and the variable is not needed by another equation in
'f', should be calculated from the values of 'x', 'xdot', and 't'
after the integration is ready.

You should consider this and check if you really need a DAE solver,
not an ODE solver (which a former posting of yours suggested).

HTH, Olaf