6. Symmetric and asymmetric.

Postscript,
PDF,
GIF (page 63, page 64, page 65, page 66, page 67, page 68, page 69, page 70, page 71, page 72, page 73, page 74, page 75, page 76, page 77, page 78, page 79, page 80, page 81, page 82, page 83, page 84)

After finishing Section 6 of my lecture notes I read a working paper of B. Lebrun. His results are stronger than these of my Sect. 6. The addendum presents the relevant part of his work in the form compatible with my lecture notes.

Addendum: An important argument of Bernard Lebrun.

Postscript,
PDF,
GIF (page 1, page 2)