Figure 15.4.9 (Page 1048):  This surface is a portion of Whitney's umbrella, which is an example of a self-intersecting surface.  This surface can be represented parametrically as

 ,

and is plotted in the  viewing window for   (s-resolution is 30) and for   (t-resolution is also 30).  The surface is shown below where the viewer angle with the x-axis is 55 degrees.