Figures 15 - 19

> z := x - 1/9*x^3 - 1/2*y^2;

> plot3d( z, x=-3.8..3.8, y=-3.8..3.8, orientation=[-45,45], title="Figure 15 a" );

> plot3d( z, x=-3.8..3.8, y=-3.8..3.8, style=patchcontour, orientation=[-45,45], title="Figure 15 b" );

> plot3d( z, x=-3.8..3.8, y=-3.8..3.8, style=contour, orientation=[-90,0], title="Figure 15 c" );

>

> z := y/(1+x^2+y^2);

> plot3d( z, x=-5..5, y=-5..5, orientation=[-45,45], title="Figure 16 a" );

> plot3d( z, x=-5..5, y=-5..5, style=patchcontour, orientation=[-45,45], title="Figure 16 b" );

> plot3d( z, x=-5..5, y=-5..5, style=contour, orientation=[-90,0], title="Figure 16 c" );

>

> z := -1 + cos( y/(1+x^2+y^2) );

> plot3d( z, x=-5..5, y=-3.8..3.8, orientation=[-45,45], title="Figure 17 a" );

> plot3d( z, x=-5..5, y=-3.8..3.8, style=patchcontour, orientation=[-45,45], title="Figure 17 b" );

> plot3d( z, x=-5..5, y=-3.8..3.8, style=contour, orientation=[-90,0], title="Figure 17 c" );

>

> z := exp( -(x^2+y^2)+x*y/4 );

> plot3d( z, x=-2..2, y=-2..2, orientation=[-45,45], title="Figure 18 a" );

> plot3d( z, x=-2..2, y=-2..2, style=patchcontour, orientation=[-45,45], title="Figure 18 b" );

> plot3d( z, x=-2..2, y=-2..2, style=contour, orientation=[-90,0], title="Figure 18 c" );

>

> z := exp( -(x^2+y^2)/4 ) * sin( x*sqrt(abs(y)) );

> plot3d( z, x=-5..5, y=-5..5, orientation=[-45,45], title="Figure 19 a" );

> plot3d( z, x=-5..5, y=-5..5, style=patchcontour, orientation=[-45,45], title="Figure 19 b" );

> plot3d( z, x=-5..5, y=-5..5, style=contour, orientation=[-90,0], title="Figure 19 c" );

>