As of March 2010 this strategy has been deprecated. It is extremely rare considering the alternatives - and perhaps because my algorithm for finding it was too specific. I would like to document it however, since it is unusual Uniqueness strategy..
The principle behind BUG is the observation that any sudoku where all remaining cells contain just two candidates is fatally flawed. There would have been a last remaining cell with three candidates. The odd number that couldn't be paired with another cell would have to be the solution for that cell in order to prevent the bi-value 'Graveyard'.
Update June 2015
Thanks to Peter Hopkins for re-engaging me with BUG. He has found the original discussion which goes back to November 2005. Here is the link
Here is an example written up by Peter
The BUG cell is D8.
Removing candidate 1 from the cell does not create a deadly pattern, since candidate 1 would appear in Row D, Column 8 and Box 6 just once. Removing candidate 2 results in:
Row D containing candidates 1, 2, 3, 4 and 8 all exactly twice.
Column 8 containing candidates 1, 2, 3 and 4 all exactly twice.
Box 6 containing candidates 1, 2, 3 and 4 all exactly twice.
Every other unit containing unsolved cells in which all candidates appear exactly twice.
Thus, in order to kill the BUG, D8 must be 2. The solver will instead find an XY-Chain.
Article created on 11-April-2008. Views: 44366 This page was last modified on 13-June-2015. All text is copyright and for personal use only but may be reproduced with the permission of the author.
Copyright Andrew Stuart @ Syndicated Puzzles Inc, 2015