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Solving Techniques 12
BUG+1
(Binary Universal Grave + 1)

BUG situation

Please look at the diagram below. [12][12] are in rows B and D.

If [1] is in B1, the result is as follows.

If [2] is in B2, the result is as follows.

In this strong link, two results can happen and both could work as the solution.
This means that there are two possible solutions to the puzzle. A Sudoku problem can’t have two solutions. This is what is known as the BUG state.

BUG+1

Below, there is the BUG from above, however, with one cell with three candidates [123]. (It is a plus one to the BUG state and hence is called BUG+1) If [1] or [2] goes into these cells, it will be in the unsolvable state like before, so in this case a [3] is entered here.

In this way, if there is a candidate that can get you out of a BUG state, using that number instead of the others, is called the BUG+1 technique.
There are more complex versions of the BUG state. Whenever there are the same two numbers to choose that, form a strong link, it is a BUG state.

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More elaborate BUGs

When looking up BUGs, a crazy sample was found posted in the comment section by someone on http://www.sudokuwiki.org/BUG, so we examined it.

If the red cell is a [1]

If the red cell is a [4]

If the red cell is an [8]

Astonishingly, in each situation the solution works. It’s amazing what some people find!

Strong link

As shown below, a link where only two numbers are possible in a row, column or box, is called a strong link. In the diagram, it is between [1] and [2].

Names of cells in Sudoku

A1A2A3A4A5A6A7A8A9
B1B2B3B4B5B6B7B8B9
C1C2C3C4C5C6C7C8C9
D1D2D3D4D5D6D7D8D9
E1E2E3E4E5E6E7E8E9
F1F2F3F4F5F6F7F8F9
G1G2G3G4G5G6G7G8G9
H1H2H3H4H5H6H7H8H9
I1I2I3I4I5I6I7I8I9

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