Sudoku Tutorial
BUG.jpg BUG stands for Bivalue Universal Grave.  It is similar to a deady rectangle in that it involves avoiding a multiple solution situation.

A BUG exists when all remaining cells:
  • are bivalue
  • all candidates exist twice in their respective houses
When this occurs, there is more than one solution so the idea is to avoid a BUG situation.

This technque is also known as BUG+1 indicating that one cell has an extra candidate.  This becomes the target cell.

The cell in row 4 col 5 has three candidates (4 8 9).  Of those candidates, each is present twice in each house except for the 9 which is present three times in row 4, col 5 and box 5.  If the 9 in row 4 col 5 were eliminated, the BUG would be complete and the puzzle would be invalid.

Therefore the target cell must be a 9.
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