When I run this piece of code:
dx=4/(nx-1); dy=2/(ny-1); phinew(2:ny-1,2:nx-1)=((dy^2*(phiold(1:ny-2,2:nx-1)+phiold(3:ny,2:nx-1))+dx^2*(phiold(2:ny-1,1:nx-2)+
