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UR - which type? Look, it's a weasel

 
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nataraj



Joined: 03 Aug 2007
Posts: 1029
Location: Vienna, Austria

PostPosted: Wed Jul 30, 2008 3:26 pm    Post subject: UR - which type? Look, it's a weasel Reply with quote

I did some reading, both here and in the sudoku player's forum but could not find a mention of this UR elimination. It does appear however in a post by ravel on Thu Jun 19, 2008 in the "ending proves challenging" thread.

Code:

+-----------+-------
|  ab .  .  |  .  abx  ... no other "a"s in row 1
|  ab .  .  |  .  aby 
|  .  .  .  |  .  . 
+-----------+--------


This is a type 3 in that there are two "floor" cells in a line (col1) and a,b plus other candidates in r12c4. The type 3 method would be to regard {x,y} as a "virtual" cell and see if anything can be done either in col 4 or in box 2. If the strong link were in col 4, we'd have a type 4.

But in any case, since there is a strong link on "a" in row 1, we can remove b from r2c4.
(r2c4=b =>r2c1=a=>r1c1=b=>r1c4=a which is DP)

This type 3 "plus strong link", is it a special type, does it have a name?

It seems to me that all types after type 4 use diagonal pairs...?


Last edited by nataraj on Tue Aug 05, 2008 8:22 pm; edited 1 time in total
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wapati



Joined: 10 Jun 2008
Posts: 472
Location: Brampton, Ontario, Canada.

PostPosted: Wed Jul 30, 2008 8:31 pm    Post subject: Reply with quote

Scanraid call it a hidden UR type 2.

http://www.scanraid.com/Hidden_Unique_Rectangles

Jean-Christophe, author of jsudoku, calls it a hidden UR.

Smile
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tlanglet



Joined: 17 Oct 2007
Posts: 2461
Location: Northern California Foothills

PostPosted: Fri Aug 01, 2008 11:09 pm    Post subject: Reply with quote

Nataraj,

I recently had a similar question on a Type 2 UR here

At my current stage of understanding, I just accept these very specific circumstances as variations on the basic type.

Ted
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nataraj



Joined: 03 Aug 2007
Posts: 1029
Location: Vienna, Austria

PostPosted: Sun Aug 03, 2008 5:21 pm    Post subject: Reply with quote

Thank you wapati and Ted!

Seems I must wait and see what other nomenclature will evolve over time.

Right now I am not happy with either "hidden type 2" or "type 2 ..." whatever. The situation is no type 2 at all here: x and y can be different and/or represent any number of additional candidates, while "type 2" in my book covers the very special case when both top cells have exactly 3 candidates and the extra candidate the same in both top cells. Maybe Andrew Stuart means "hidden UR type 2" in the sense that there are two types of hidden URs. Myself I do not like the notion of "hidden" UR - what kind of other UR is there?

Be that as it may, I'll just have to call these babies "UR ab r12c15 plus strong link on a in row 1" ... That is just as well, since usually the shorthand descriptions require more specific attributes anyway, think of "W-wing 18 with strong link on 8 in row 3", "kite (2) in row 4 and col 9".

Thanks again!
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tlanglet



Joined: 17 Oct 2007
Posts: 2461
Location: Northern California Foothills

PostPosted: Sun Aug 03, 2008 6:15 pm    Post subject: Reply with quote

nataraj wrote:


Be that as it may, I'll just have to call these babies "UR ab r12c15 plus strong link on a in row 1" ... That is just as well, since usually the shorthand descriptions require more specific attributes anyway, think of "W-wing 18 with strong link on 8 in row 3", "kite (2) in row 4 and col 9".

Thanks again!


I agree completely! My hints for the August 3 VH puzzle did exactly that, eg,
1) A kite on <7> in box 1
2) A Type 4 UR on <57> in r49c89 with a strong link on <5> in row 4

Ted
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nataraj



Joined: 03 Aug 2007
Posts: 1029
Location: Vienna, Austria

PostPosted: Tue Aug 05, 2008 8:22 pm    Post subject: Reply with quote

Since no better suggestions turned up, I'll go and call these still unnamed patterns

UR "With Strong Link" or UR wsl (pronounced weasel).



There are two types of UR weasels:

a) the "straight weasel": a pair of bi-value cells (a,b) on the SAME line (the "floor" cells). The strong link (let's say, in "a") is in a line perpendicular to the floor (NB if the strong link is parallel to the floor it is a type 4).

In this weasel, "b" (not a!) can be removed from the multi-value cell that is not part of the strong link.
Code:

ab   ab
--------
abx  a-by
 .
 . strong link a in column
 .



b) the "cross weasel": the bi-value cells are at diagonally opposite corners. There is a strong link in "a" at any one of the four edges of the rectangle.

In this weasel, "a" (the candidate which forms the strong link) can be removed from the multi-value cell that is not part of the strong link.
Code:

abx  ab
--------
ab   -aby
 .
 . strong link a in column
 .


Last edited by nataraj on Tue Aug 05, 2008 8:59 pm; edited 1 time in total
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nataraj



Joined: 03 Aug 2007
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Location: Vienna, Austria

PostPosted: Tue Aug 05, 2008 8:25 pm    Post subject: Reply with quote

And the well known type 6 (x-wing on a, which just means TWO parallel strong links, therefore remove a from BOTH multi-value cells) would be a double cross weasel then... Laughing Laughing Laughing
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