c function A = lap2D_dens(nx,ny)
% function A = lap2D_dens(nx,ny)
%--- generates a 2-D laplacean --------------
%%  dense 
tx = tridiag(-1, 2, -1, nx) ;  
ty = tridiag(-1, 2, -1, ny);
A  = kron(eye(ny,ny),tx)+kron(ty,eye(nx,nx)); 
