Date:  4 NOV 1980 0023-EST
From: DCP at MIT-MC (David C. Plummer)
Subject:  Christman Cross algorithm described
To: CUBE-HACKERS at MIT-MC


Let the following notation exist:
the faces:
	T=Top	B=Bottom
	F=Front	P=Posterior (Rear, Back, but those letters are taken)
	R=Right	L=Left
The rotations:
	+ Clockwise
	- Clockwise
	++ 180 Rotation (Two moves)

Note that the following algorithm exchanges corners diagonally on
top and bottom: (Parens for major functional components)

	(+F+P)(++T++B)(-F-P)(-R-L)(++T++B)(+R+L)(++T++B)

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 

	(+F)(++T++B)(-F-P)(-R-L)(++T++B)(+R+L)(++T++B) +P

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.