x is f in the article A is the five-point laplacian (a symmetric matrix with five non-zero diagonals), as implemented in the "step" function in the article, le

x is f in the article A is the five-point laplacian (a symmetric matrix with five non-zero diagonals), as implemented in the "step" function in the article, let's call it L b is the datum h If you set-up the sparse matrix L and the vector b (with the same code that performs the iterations in the article), then you can solve Poisson equation "L*f=h" by doing "f=L\h".