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5 JUne difficult -has this technique got a name?
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jLo



Joined: 30 Apr 2007
Posts: 55

PostPosted: Thu Jun 07, 2007 4:02 am    Post subject: Reply with quote

TexCat wrote
Quote:

I certainly agree that there is no need for w-wings on this puzzle. Dulaby's xy-wing solves it nicely. I cannot see any skyscraper on 1's though.


I see the XY-Wing, but only after row 3 is cleaned up. I get to here -

Code:

+---------------+-----------------+---------------+
| 17  6    378  | 1478  5    48   |  49  2    39  |
| 5   18   238  | 9     146  2468 |  7   146  36  |
| 9   4    27   | 3     167  26   |  8   16    5  |
+---------------+-----------------+-------------- |
| 6   5    1    | 2     8    7    | 3    9    4   |
| 8   2    9    | 46    46   3    | 1    5    7   |
| 3   7    4    | 5     9    1    | 26   68   268 |
+---------------+-----------------+-------------- |
| 24  38   5    | 468   346  9    | 246  7    1   |
| 247 138  6    | 478   1347 5    | 249  48   289 |
| 147 9    78   | 14678 2    468  | 5    3    68  |
+---------------+-----------------+---------------+


How do you go about solving Row 3 to set up the XY-Wing? The best I could
do was see that if R3C3=7 then R7C2=3 and R7C5=3, so R3C3 cannot be 7 . . .
Is there another method?

I don't see the Skyscraper either.
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Mogulmeister



Joined: 03 May 2007
Posts: 1151

PostPosted: Thu Jun 07, 2007 10:17 am    Post subject: Reply with quote

I don't see a skyscraper on 1's but perhaps cg meant x-wing on 1's which I see you've already applied j-Lo. (Although that doesn't remove the 1 in r2c5)

Have a look at row 8 and you'll see that there is a hidden pair. This will create a single on row 3 which in turn will remove the 2 in r2c3. You can now build that xy wing.
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cgordon



Joined: 04 May 2007
Posts: 769
Location: ontario, canada

PostPosted: Thu Jun 07, 2007 6:05 pm    Post subject: Reply with quote

I always thought skyscrapers could be horizontal as well as vertical - they are if you rotate the page! Maybe vertical skyscrapers are y wings and horizontal ones are x wings. Anyway, using George's grid on his first post, there are two 1's in col. 2 and two in col 8. By my reckoning since the two 1's in Row 2 line up. one of these has to be a 1. Therefore the 1 in R2C5 is eliminated. No???
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Mogulmeister



Joined: 03 May 2007
Posts: 1151

PostPosted: Thu Jun 07, 2007 6:58 pm    Post subject: Reply with quote

cg,

Yes skyscaper can be vertical or horizontal. However they are strong links of different length that both have a node on the same row/column.

I see where you're driving at but your row 2 has actually got 3 boxes with 1 in it so it is not a strong link. The other problem is that for a candidate to be eliminated it must be able to be "seen" by the two ends of the skyscraper.

cg - if you want to see a graphic example of a skyscraper look at the one that is illustrated in Mesmin's puzzle in this thread
[url]
http://www.dailysudoku.com/sudoku/forums/viewtopic.php?t=1860&sid=9cb62644c9357068f48772c4a472f413 [/url]

The skyscraper is in pink, the eliminated 5's in orange.
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csc



Joined: 03 May 2007
Posts: 14

PostPosted: Thu Sep 20, 2007 3:23 pm    Post subject: 5 jun 07 most straightforward solution Reply with quote

I've just completed this puzzle, after having read the discussion on it. I was at the same position as George, when I got stumped. Seems to me that the most straightforward route is as suggested by Steve R. After having identified the 13 hidden pair, and getting the 7 in r3c5, you need only 1 xy wing (to eliminate 8), to complete. Thanks Steve.
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