Advanced methods and approaches for solving Sudoku puzzles

Re: Forcing chains / forcing nets

Jeff wrote:Sorry, I missed the xy-chain in the first snippet. How does it go? The chain I found is as follows:

r3c4=4 => r2c4=23 which forms a naked pair with r2c3 => r2c2=1
r3c4=5 => r3c3=3 => r2c3=2 => r2c2=1

Here's what I had in mind ...
Code: Select all
`Snippet #1           |   12  23 |  24        35 |  45 ----------+----------contains the forcing pattern (an xy-chain)   23---35   /      | 12       |   \      |    24---45 `

Code: Select all
`Snippet #2             |   12  23 | 234        35 |  45 ----------+----------contains the forcing pattern (a "net")   23---35   / |    | 12  |    |   \ |    |    234--45 `

Both patterns force exclusion of the '2' in '12' -- any one of the other four cells can be used as the "forcing cell" (you chose the '45').
r.e.s.

Posts: 337
Joined: 31 August 2005

Re: Forcing chains / forcing nets

r.e.s. wrote:It isn't clear to me why the forcing net (a *chain*) contained in the first snippet would be considered an acceptable "target pattern", but not so for the forcing net in the second snippet. With respect to "recognisability" of these two patterns, their difference seems only a matter of (small) degree.

Thanks for clearing this up. I thought the description 'first snippet' meant the original snippet (which in fact is the second snippet), and missed out the obvious xy-chain.
Jeff

Posts: 708
Joined: 01 August 2005

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