I have a differential equation in the form: Uxx + Uyy + c*U = f(x,y).
where Uxx is the 2nd derivative of U and so Uyy.

I want to solve the problem using 2-D collocation in the square [0,1]x[0,1].
I merge this square in 16 equal squares (N=4 for each axe).
So i have to solve the A*q=B.

Which element is the A(10,10) ?