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September 18th Very Hard

 
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David



Joined: 02 Jun 2006
Posts: 58
Location: Bedford, UK
PostPosted: Thu Sep 21, 2006 4:59 pm    Post subject: September 18th Very Hard Reply with quote
It seems very quiet on the board at the moment so perhaps I can clutter it up with comment on the last v hard puzzle.

I had quite a bit of difficulty spotting the X wing on 3's. I filled in 9 numbers using 'easy' techniques, but the X wing eluded me - too many candidates to deal with. Anyway, giving up my X wing spotting, I thought I would try 'strong links'. Eventually I saw one on 3's using c 9, r2,9 and r 3, c5,7. This allows (I think) elimination of the 3 (leaving a 7) in the cell at r9 c5. As I started to remove the other 7's from c5 I spotted the X wing using half of my 'strong link' pattern and cursed myself for missing something so obvious.

Anyway, is the 'strong link' pattern also referred to as a kite ? or is it a turbot fish ? I am slowly starting to become familiar with other 'advanced' techniques, but the terminology is very confusing when you read comments on other Sudoku boards.
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA
PostPosted: Fri Sep 22, 2006 5:08 am    Post subject: Reply with quote
In that puzzle, I used a Type 4 rectangle on 15, then the X-Wing on "3" solved the puzzle.

Quote:
I had quite a bit of difficulty spotting the X wing on 3's.


It is rare that I can "spot" an X-Wing; I have to do it methodically. When I've exhausted other possibilities and have to start looking for X-Wings, I start in row 1 looking for strong links. Each time I find one, I write the number down outside the grid and scan the other rows to see if the same strong link exists in the same two columns for the X-Wing.

After I've done all nine rows, I then look at the rows with the same strong links to see if the strong link (fork or skyscraper) technique can be used to eliminate candidates. Then, if need be, I repeat the procedure with columns.
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ravel



Joined: 21 Apr 2006
Posts: 536
PostPosted: Fri Sep 22, 2006 8:42 am    Post subject: Reply with quote
Code:
37  135   1257  | 6   #2357  579   | 4     #239    8         
48  345   2578  | 39   1     579   |#2379   6     c23         
9   6     27    | 4   a237   8     |a237    5      1     
-----------------------------------------------------   
6   7     3     | 8    49    2     | 19     149    5         
2   49    89    | 5    6     1     | 38     348    7         
48  15    15    | 7    49    3     | 6      2489   24       
-----------------------------------------------------     
1   8     4     | 2   b35    6     |b35     7      9         
37  2     79    | 139  8     459   |#135   #134    6         
5   39    6     | 139 #37    479   |#1238  #12348 c234   

Alternatively to using the x-wing the 3 in r9c5 can be eliminated with 2 strong links, which is (essentially) the same as a turbot fish:
r3c5-r3c7,r2c9-r9c9 (either r3c5 or r9c9 must be 3).

You also can use 3 strong links ("Advanced coloring" is the same as n strong links):
r3c5-r3c7,r2c9-r9c9,r7c7-r7c5 (either r3c5 or r7c5 must be 3 - eliminates 3 in r1c5,r9c5).

An x-wing is a special case of 2 strong links, e.g. r3c7-r3c5,r7c5-r7c7 eliminates 3 in r289c7.

With the 3 strong links r3c7-r3c5,r7c5-r7c7,r9c9-r2c9 you could eliminate the 2 marked 3's in box 3.
Using another order (starting with r3c7 to r9c9) those in box 9.

I dont remember, what Havard called a kite.
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David



Joined: 02 Jun 2006
Posts: 58
Location: Bedford, UK
PostPosted: Fri Sep 22, 2006 4:50 pm    Post subject: Reply with quote
Quote:
Alternatively to using the x-wing the 3 in r9c5 can be eliminated with 2 strong links, which is (essentially) the same as a turbot fish:
r3c5-r3c7,r2c9-r9c9 (either r3c5 or r9c9 must be 3).

Hi Ravel

This is what I used, but you answered my question in naming it a 'turbot fish'. I have never seen 3 strong links used together so I will study your solution here.
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TKiel



Joined: 22 Feb 2006
Posts: 292
Location: Kalamazoo, MI
PostPosted: Sat Sep 23, 2006 12:02 pm    Post subject: Reply with quote
David,

This thread contains some examples and real life puzzles where three strong link chains were used together.
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David



Joined: 02 Jun 2006
Posts: 58
Location: Bedford, UK
PostPosted: Sat Sep 23, 2006 3:10 pm    Post subject: Reply with quote
Hi Tracey

Thanks for the pointer I will look it up. The more examples, the easier it is to check you have understood it correctly.
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