Figures 6 - 9
> z := -4*x^3*y^2;
> plot3d( z, x=-1..1, y=-1..1, title="Figure 6" );
>
> z := x - 1/8*x^3 - 1/3*y^2;
> plot3d( z, x=-4..4, y=-4..4, title="Figure 7" );
>
> z := x*y * exp( -x^2-y^2 );
> plot3d( z, x=-3..3, y=-3..3, title="Figure 8" );
>
> z := exp(-abs(x)) * cos( sqrt(x^2+y^2) );
> plot3d( z, x=-5..5, y=-5..5, title="Figure 9" );
>