Solver App for Android and iPhone
Strategies for Number Puzzles of all kinds
Page Index
Basic Strategies
Tough Strategies
Diabolical Strategies
Extreme Strategies
Deprecated Strategies
Order Str8ts Book 1
Order Now! Order Str8ts Book 2
Order Now!


BUG stands for Bi-Value Universal Grave

As of July 2015 this strategy has been re-instated in the solver..

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 July 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. From my testing of large data sets I believe that every instance of BUG can be solved by an XY-Chain. Hence it is positioned just before that strategy in the solver - it is an easy solution if you can recognise the pattern. Other simpler strategies may also do the same job but not as completely as XY-Chains.

BUG Example
BUG Example : Load Example
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:
  1. Row D containing candidates 1, 2, 3, 4 and 8 all exactly twice.
  2. Column 8 containing candidates 1, 2, 3 and 4 all exactly twice.
  3. Box 6 containing candidates 1, 2, 3 and 4 all exactly twice.
  4. Every other unit containing unsolved cells in which all candidates appear exactly twice.

Thus, in order to kill the BUG, D8 must be 2.

BUG Exemplars

These puzzles require the Bi-Value Universal Grave strategy at some point.
Only the first is somewhat trivial. They make good practice puzzles.


Your Name/Handle

Email Address - required for confirmation (it will not be displayed here)

Your comment or question

Please enter the
letters you see:
Enter these letters Remember me

Please keep your comments relevant to this article.
Email addresses are never displayed, but they are required to confirm your comments. When you enter your name and email address, you'll be sent a link to confirm your comment. Line breaks and paragraphs are automatically converted - no need to use <p> or <br> tags.
Talk Subject Comments
Comments here pertain to corrections to the text, not the subject itself
Article created on 11-April-2008. Views: 89458
This page was last modified on 1-May-2016.
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, 2016