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.

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'.

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.

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:

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

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.

## Comments

Comments Talk## Wednesday 11-Mar-2015

## ... by: Brett Yarberry

I have found a good example of a solution easily solved by the BUG.The initial position of the board (loaded in solver) is: LOAD HERE

## Thursday 15-Mar-2012

## ... by: Arthur Lurvey

When you write up this technique, consider using the following exampleI got it from http://homepages.cwi.nl/~aeb/games/sudoku/solving18.html. It can be solved using other methods, but they are of the diabolical class. So this makes for a good application of this technique.

Art

## Tuesday 17-May-2011

## ... by: Peru Boro

Does Bug+1 system always work? Usually it does work but twice it failed me so I want to be sure.Thank you.## Wednesday 24-Mar-2010

## ... by: Sean Forbes

Andrew, While I agree that a more astute player will more than likely identify and utilize some other solving technique before resorting to this one, I find this method very handy in real-time online competitions, especially if I've overlooked one of the more effective solving techniques up to the point when I can recognize the BUG pattern.Thanks. Sean

## Friday 19-Mar-2010

## ... by: Harmen Dijkstra

i have a sudoku with this strategy:+--------------+--------------+--------------+

| 4 2 1 | 5 8 6 | 9 3 7 |

| 5 9 38 | 1 34 7 | 2 48 6 |

| 6 7 38 | 2 34 9 | 14 148 5 |

+--------------+--------------+--------------+

| 7 1 9 | 8 2 3 | 5 6 4 |

| 2 5 6 | 4 7 1 | 8 9 3 |

| 8 3 4 | 9 6 5 | 17 17 2 |

+--------------+--------------+--------------+

| 1 6 5 | 3 9 2 | 47 47 8 |

| 3 4 2 | 7 1 8 | 6 5 9 |

| 9 8 7 | 6 5 4 | 3 2 1 |

+--------------+--------------+--------------+

with this strategy, we will get this solution:

+--------------+--------------+--------------+

| 4 2 1 | 5 8 6 | 9 3 7 |

| 5 9 3 | 1 4 7 | 2 8 6 |

| 6 7 8 | 2 3 9 | 1 4 5 |

+--------------+--------------+--------------+

| 7 1 9 | 8 2 3 | 5 6 4 |

| 2 5 6 | 4 7 1 | 8 9 3 |

| 8 3 4 | 9 6 5 | 7 1 2 |

+--------------+--------------+--------------+

| 1 6 5 | 3 9 2 | 4 7 8 |

| 3 4 2 | 7 1 8 | 6 5 9 |

| 9 8 7 | 6 5 4 | 3 2 1 |

+--------------+--------------+--------------+

However, there are other solutions, for example:

+--------------+--------------+--------------+

| 4 2 1 | 5 8 6 | 9 3 7 |

| 5 9 8 | 1 3 7 | 2 4 6 |

| 6 7 3 | 2 4 9 | 1 8 5 |

+--------------+--------------+--------------+

| 7 1 9 | 8 2 3 | 5 6 4 |

| 2 5 6 | 4 7 1 | 8 9 3 |

| 8 3 4 | 9 6 5 | 7 1 2 |

+--------------+--------------+--------------+

| 1 6 5 | 3 9 2 | 4 7 8 |

| 3 4 2 | 7 1 8 | 6 5 9 |

| 9 8 7 | 6 5 4 | 3 2 1 |

+--------------+--------------+--------------+