dailysudoku.com Forum Index dailysudoku.com
Discussion of Daily Sudoku puzzles
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

A learning experience

 
Post new topic   Reply to topic    dailysudoku.com Forum Index -> Other puzzles
View previous topic :: View next topic  
Author Message
keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Sat Oct 27, 2007 9:34 pm    Post subject: A learning experience Reply with quote

LA Times 040606
Code:
+-------+-------+-------+
| . . . | . . 2 | . 1 . |
| . . 4 | . 3 1 | . 8 6 |
| . 5 . | 4 . . | 3 . . |
+-------+-------+-------+
| . . . | . 2 . | 4 . 9 |
| . . . | 6 . 4 | . . . |
| 1 . 2 | . 9 . | . . . |
+-------+-------+-------+
| . . 9 | . . 6 | . 3 . |
| 2 6 . | 5 1 . | 9 . . |
| . 8 . | 2 . . | . . . |
+-------+-------+-------+

I found this in another discussion. re'born points out an incredible move!

Keith


Last edited by keith on Sun Oct 28, 2007 11:16 am; edited 1 time in total
Back to top
View user's profile Send private message
Asellus



Joined: 05 Jun 2007
Posts: 865
Location: Sonoma County, CA, USA

PostPosted: Sun Oct 28, 2007 1:47 am    Post subject: Reply with quote

I have no idea what re'born's "incredible move" is, but I like a nice challenge. So, I found a very interesting DP solution. Basics get one here, I believe:
+-----------------+--------------+-----------+

| 3689 39 368 | 89 5 2 | 7 1 4 |
| 79 2 4 | 79 3 1 | 5 8 6 |
| 78 5 1 | 4 6 78 | 3 9 2 |
+-----------------+--------------+-----------+
| 35678 37 3568 | 1 2 578 | 4 57 9 |
| 5789 79 58 | 6 78 4 | 1 2 3 |
| 1 4 2 | 3 9 57 | 8 6 57 |
+-----------------+--------------+-----------+
| 45 1 9 | 78 478 6 | 2 3 57 |
| 2 6 7 | 5 1 3 | 9 4 8 |
| 345 8 35 | 2 47 9 | 6 57 1 |
+-----------------+--------------+-----------+

The potential DP is in red. Note that the <4>s form an X-Wing.

If R4C6 is <5>, the DP results. But also, if R4C6 is <7>, the DP results. (R5C5 would be <8> and R9C3 would be <3>, if you need help seeing this.) So, R4C6 must be <8>.

This solves the puzzle.
Back to top
View user's profile Send private message Visit poster's website
keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Sun Oct 28, 2007 2:11 pm    Post subject: Reply with quote

What is a DP?

Basics get you here:
Code:
+----------------+----------------+----------------+
| 3689 39   368  | 89   5    2    | 7    1    4    |
| 79*  2    4    | 79@  3    1    | 5    8    6    |
| 78   5    1    | 4    6    78@  | 3    9    2    |
+----------------+----------------+----------------+
| 368  37   368  | 1    2    578@ | 4    57   9    |
| 58-9 79*  58   | 6    78@  4    | 1    2    3    |
| 1    4    2    | 3    9    57@  | 8    6    57   |
+----------------+----------------+----------------+
| 45   1    9    | 78   478  6    | 2    3    57   |
| 2    6    7    | 5    1    3    | 9    4    8    |
| 345  8    35   | 2    47   9    | 6    57   1    |
+----------------+----------------+----------------+

The cells marked * work like a W-wing to remove <9> from R5C1, which solves the puzzle. The coloring chain marked by @ is unique in that the two cells with candidate <7> in B5C6 act like a single cell. Try it!.

If R2C1 is <7>, R5C2 is <9>.
If R5C2 is <7>, R2C1 is <9>.

One or both of R2C1 and R5C2 is <9>, which makes the elimination.

re'born calls this a semi remote pair. I have lost his post for the moment, and will look for it.

Keith
Back to top
View user's profile Send private message
storm_norm



Joined: 18 Oct 2007
Posts: 1741

PostPosted: Sun Oct 28, 2007 3:31 pm    Post subject: Reply with quote

disjointed pair?
Back to top
View user's profile Send private message
nataraj



Joined: 03 Aug 2007
Posts: 1048
Location: near Vienna, Austria

PostPosted: Sun Oct 28, 2007 3:49 pm    Post subject: Reply with quote

DP = deadly pattern.

I think what box 5 does is something like an ER (empty rectangle).
If r3c6=7 then none of c6=7 then r5c5=7.
Your chain, keith, looks to me like a grouped AIC where a group of cells act like one single cell for the purpose of the chain.
Back to top
View user's profile Send private message Send e-mail Visit poster's website
nataraj



Joined: 03 Aug 2007
Posts: 1048
Location: near Vienna, Austria

PostPosted: Sun Oct 28, 2007 3:54 pm    Post subject: Reply with quote

BTW, I did a coloring elimination of '8' in r5c1 first (probably unnecessary), then used an xy-chain (one of many, many, many many ... xy chains available) to remove 5 from r4c6 and r6c9:

(r4c8=7)=>(r4c2=3)=>(r1c2=9)=(r1c4=8 )=>(r3c6=7)=>(r6c6=5)
Back to top
View user's profile Send private message Send e-mail Visit poster's website
keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Sun Oct 28, 2007 4:42 pm    Post subject: Reply with quote

Deadly Pattern. I see it now.

A slightly different way to argue it is that to avoid the DP, R4C6 is <8>, and / or R7C5 is <8>, and / or R9C1 is <3>. Either of these forces the X-wing to have <4> in R7C1 and R9C5. This is not as direct as Asellus' solution, for you still need a (real) remote pair to finish it up.

You can also argue that each of R7C5 is <8>, R9C1 is <3>, force R4C6 to be <8>. Therefore, R4C6 must be <8>. R7C5 is easy, R9C1 is much longer, almost equivalent to solving the puzzle. (In the solution, all of the DP busters are true: R4C6 is <8> and R7C5 is <8> and R9C1 is <3>.)

Keith
Back to top
View user's profile Send private message
keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA

PostPosted: Wed Oct 31, 2007 11:06 pm    Post subject: Reply with quote

From re'born, on the history:

re'born wrote:
keith wrote:
re'born:

http://www.dailysudoku.com/sudoku/forums/viewtopic.php?t=2107

Can you help me find your original post of this puzzle?

Here you go:
http://www.sudoku.com/boards/viewtopic.php?p=48056#48056

This post contains a link to the thread in which I originally found the puzzle.

keith wrote:

I presume that the idea of a semi-remote pair has been around longer than the W-wing?

I'll try to give you the history as I know it. The first recorded use of a w-wing that I know of is John Macleod in Jan. 2006, who called it the Corkscrew Rule:
http://www.sudoku.org.uk/cgi-bin/discus/show.cgi?tpc=29&post=2790#POST2790
His rule was overly restrictive and I pointed this out formulating the general rule. I subsequently forgot about this exchange and didn't check for these things.

In March of this year, Stephen K posted his Y-wing styles thread which used the w-wing (not called that) as a main example of his more general notion. I posted often in this thread and suggested that the specific technique be called a semi-remote naked pair. This same has become semi-standard (in the sense that people usually call the technique several names when they use it and semi-remote naked pair is one of these names).

At some point in August, you created the thread on W-wings and gave some very cool corollaries of the rule.

So, in principle, the w-wing/semi-remote naked pair/y-wing style are all the same. In practice, I've only seen w-wings used in the way xy-wings are used, while I use semi-remote naked pairs allowing for longer chains in between the endpoint pairs.

By the way, I recently posted what I think is a rather slick application of semi-remote naked pairs in a solution to one of wapati's puzzles.
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    dailysudoku.com Forum Index -> Other puzzles All times are GMT
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group