plot3d (log(x^2*y^2), [x,-2,2], [y,-2,2], [z,-8,4], nopalette, [color,magenta],
  [pdf_file,"./plotting18.pdf"]);
plot3d (log(x^2*y^2), [x,-2,2], [y,-2,2], [z,-8,4], nopalette, [color,magenta],
  [png_file,"./plotting18.png"]);
