Search for Pairs of Numbers
It's easiest to describe the search of pairs of numbers by an example:
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- A sparse Sudoku
This Sudoku does not have a uniq solution, but it is a good example nontheless.
When you try normal crosshatching you don't find a
number, but you find that the 2 and 8 have
only two possible cells in the upper left block.
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The
2and8have only two possible cells each.
You should mark this in your Sudoku because it is usefull later on.
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- The places for the two numbers are marked.
The 2 and 8 have to be in these two
cells, therefore no other numbers can be in these cells.
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- The two cells are blocked for all other numbers.
Knowing that you can again use crosshatching:
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You can find a place for the
4using crosshatching.
The 4 previously had three possible positions in the
upper left 3x3 block, now only of of them is left:
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The Sudoku with the newly found
4
This technique is very usefull when using pen and paper to solve a Sudoku.
It works with triples instead of pairs as well, if there the same three cells are possible for all three numbers.
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