Let the following notation exist:
F=Front P=Posterior (Rear, Back, but those letters are taken)
++ 180 Rotation (Two moves)
Note that the following algorithm exchanges corners diagonally on
top and bottom: (Parens for major functional components)
This is now a tool. To do the Christman Cross, we do
-P [(+F+P)(++T++B)(-F-P)(-R-L)(++T++B)(+R+L)(++T++B)] +P
but since transforms associate, and F and P commute, and -P+P = I, we get
Which is 20 quarter twists.
David Christman thinks he read in Singmaster, Edition 5 that this
cross can be done in less than 20 quarter twists. Can somebody
confirm or deny this recolection.