Solving a large system of equations - symbolic problem

[Mohammed Ahmed]

We have been using Octave successfully to solve linear & non-linear 
systems of differential equations.  However, we have now encountered 
a situation as follows:
We use lsode, and the xdot(n) = function of (y,u).
y is a large system of equations (300) in the form:
y(i-1) + 4*y(i) + y(i+1) = 3 (u(j+1) - u(j-1))
... where n = 1-100, i = 1-300, j =1-100.

The above is an example, this is not a geneal expression for all
equations in the system.

Octave requires that equations be expressed as follows:
y(i) = function of (u(1) ... u(2) .... u(j))    ; where j=1-100\

How do we get our system of equations into the required form.  
Can Octave do this for us?  Anyone
out there have a computer based method?
Suggestions?

... Tx .. Mahammad