Solving DAE equations simultaneously with ODE15s

Hi,

I have a set of equations as DAE which I need to be solved. I am using ODE15s. The equations are of the form.

(1/y)(dx/dz)+(2x)(dy/dz)=q; q+y+5=0; (I/x)(dp/dz)=y; (dx/dz)+(dy/dz)=25; where x,y,p and q are the variables needed to be solved as initial value problem. Can you please help how to proceed as from example here (https://www.mathworks.com/help/matlab/ref/ode15s.html) I can only see one derivative in one equation and that is why the M matrix is very simple. How can I write the M matrix in my case? If I need to solve similar problem as boundary value problem, is it possible? Thanks...

Answer 1

[Torsten]

M=[1/w(2) 2*w(1) 0 0; 0 0 0 0; 0 0 l/w(1) 0; 1 1 0 0] f=[w(4);w(4)+w(2)+5.0;w(2);25.0]

if you order the solution variable as w(1)=x, w(2)=y, w(3)=p, w(4)=q.

If you solve your differential equations explicitly for dx/dz, dy/dz and dp/dz, it is possible to use "bvp4c". This is easily possible for your system from above.

Best wishes

Torsten.