I am dealing with following 2-dimensional problem in the unit square domain $S_2$
$$- \Delta u (x,y) = f \ \text{in} \ S_2, \hspace{1.5cm} u(x,y) = 0 \ \text{on} \ \partial S_2$$
where $f$ is such the analytical solution to problem is
$$u(x,y) = \cos [ 2 \pi x (y - x) ]^2.$$
Problem should be discretized by 5-point finite difference scheme, which will result in Ax = b system. I need to decompose problem into 4 non-overlaping domains. Should I divide my domain square into 4 parts and then assemble? or assemble and then divide? Main question here is, how the laplacians will look like.