Thu, 29 May 2003 14:02:27 -0400 Andy Latto <Andy.Latto@gensym.com> Right. Both the original proof and my reworking of it only prove the (true) statement that there is no closed knight's tour on a 4xn board, and not the (false) statement that there is no open knight's tour. An analysis of either proof shows that if there is an open knight's tour on a 4xn board, then exactly one link in the the tour connects two non-extreme squares, and that this must be the middle link of the tour. Yes; sorry. I missed the open/closed tour distinction until after I sent my message and read the msg from Fred Helenius that pointed it out. Reentrant/closed tours rule out my counterexample because you can't have both first and last moves be extreme squares, given that you can't move from extreme to extreme in one move. One more example of why I shouldn't reply promptly to email.