1 plot3d ([[-3*x-y, [x,-2,2], [y,-2,2]], 4*sin(3*(x^2+y^2))/(x^2+y^2), [x,-3,3],
2 [y,-3,3]], [x,-4,4], [y,-4,4], [pdf_file,"./plotting20.pdf"]);
3 plot3d ([[-3*x-y, [x,-2,2], [y,-2,2]], 4*sin(3*(x^2+y^2))/(x^2+y^2), [x,-3,3],
4 [y,-3,3]], [x,-4,4], [y,-4,4], [png_file,"./plotting20.png"]);
