The New Sudoku Players' Forum

[Ruud]

Why Sadman offered the 1 instead of the 9 I don't know

The reason Sadman offers the 1 is this:

There are 2 possible XY-Wings in this picture:

R1C5 {2,9} - R4C5 {6,9} - R2C6 {2,6}

and

R2C6 {2,6} - R1C5 {2,9} - R4C6 {6,9}

For some reason, Sadman preferred the latter. It eliminates candidate 9 from R4C5 (leaves 6) and R3C6 (leaves 1)

Ruud.

[Shazbot]

True, but the elimination of 9 from r3c6 can only be done AFTER it's placed in r4c6 by either XYWing - I'm assuming an XYWing can only make one elimination, which may lead to others (by your own reasoning, leave 6 in r4c5, place 9 in r4c6, THEN place 1 in r3c6). Or doesn't that program show you the individual steps (I've only used it a couple of times, and not used the hint feature)? By my reckoning there are one or two placements to be made before the 1 in r3c6 - I'm wondering why they're not shown as "next steps", or doesn't Sadman Sudoku necessarily show you the next logical step (I know Pappocom will show you a placement, but not necessarily the next logical one)

[Ruud]

I'm assuming an XYWing can only make one elimination

That assumption is not correct. An XY-Wing can make multiple eliminations, upto 5 candidates maximum AFAIK.

Here is the second XY-Wing. I've colored the XY-Wing in green, and the eliminated candidates in red. There's 2 of them, as you can see.



Ruud.

[rubylips]

Alternatively,

Code: Select all

Consider the cell r2c6.
When it contains the value 6, the value 9 in Column 6 must occupy the cell r4c6.
When it contains the value 2, the value 9 in Box 2 must occupy the cell r1c5.
Whichever value it contains, the cells r1c6 and r3c6 cannot contain the value 9.
- The move r3c6:=9 has been eliminated.
The value 1 is the only candidate for the cell r3c6.


[bjschnei]

Can you explain which cells the xy-wing is in. I am still not seeing it. Haven't solved one with that technique yet. Thanks for your help!

[bjschnei]

didn't see page 2 on the forum..never mind.