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mtharp · Joined: 19 Jun 2006 Posts: 30

Folks, immp in particular, I believe the puzzle has two URs on 6 & 9 in R2,5 C1,3 and R2,7 C1,7. I worked on the first one and since there were no other 9s in column 3, I eliminated the 6s but wound up at a contradiction.

Any help appreciated especially if it does not involve chains.

Marty tells me that some superb players use chains, but I am still struggling with patterns.

Marty R. · Posted: Mon Mar 05, 2018 12:48 am Post subject:

ZeroAssoluto · Joined: 05 Feb 2017 Posts: 941 Location: Rimini, Italy

Hi Mike,

I don't see UR too.
One way to solve it is a W-Wing 2,9 in r1c1,r7c9 connected by number 2 in r17c8 and -9 in r7c1.

Ciao Gianni

Ajò Dimonios · Joined: 01 May 2017 Posts: 339 Location: Sassari Italy

Hi everyone

[2R1C1-2R1C8=2R7C8-2R7C9 ;(2-9)R1C1=9R1C8-9R6C8=9R6C2-9R9C2=9R7C1-9R7C9]=>contradiction (R7C9=Ø)=>-2R1C1=>solution.

Ciao a Tutti
Paolo

dongrave · Joined: 06 Mar 2014 Posts: 568

(9=2)r7c9-r7c8=r1c8-(2=9)r1c1=>r7c1<>9

immpy · Joined: 06 May 2017 Posts: 571

Mike, the first UR you mentioned is vague, at best. As a Type 4 (and that is stretching the concept, indeed) one could make an elimination of the 9 in both r2c1 and r5c1. But this takes us no further along really, kind of fruitless. There are still too many 9s left to deal with.
The second one you mention I do not see at all. It's not there.

Just curious Mike...are you on a mission with URs?? LOL
You seem to be obsessed with them.
I'm not trying to be critical, or bash, or anything of the sort. Just wondering is all.
I always appreciate any comments and will try to help when I can.

cheers...imp

...and the correct logic would be to eliminate the 9s from the cells opposite of the two cells where 9 is occurring only twice in that house. The 6s should not be removed from anywhere in the UR.

There is a good explanation of the various Uniqueness Tests here:
http://hodoku.sourceforge.net/en/techniques.php

The site goes down for maintenance frequently, it seems, but when it is available it explains things quite well, I find.

mtharp · Joined: 19 Jun 2006 Posts: 30

Marty, Gianni & immp, I miss wrote the second one. I see one on 6 & 9 in R2,7 C 1,8 not C 1,7.

immp, I work the Chicago Tribune Friday sudoku - they are the one that keep coming up with them. I am just learning the concept, so I keep asking the questions. I appreciate your patience with me.

Everyone thank you.

Mike

Marty R. · Posted: Mon Mar 05, 2018 10:06 pm Post subject:

With all the UR talk here, has everyone read Keith's article on the basics?

http://www.dailysudoku.co.uk/sudoku/forums/viewtopic.php?t=3578

immpy · Joined: 06 May 2017 Posts: 571

Upon a closer look and a refresher of the concepts of URs, I would not even designate either of these as a UR of known type (the possible two 69 URs that Mike is referring to). I did see the second one you were referring to Mike, after your correction of the earlier post.
There are simply too many 6s and 9s in the houses (rows, columns, and boxes) that make up these rectangles in order for them to be present. And any extra candidates present do not seem to form a pseudo-cell with other candidates in a common house (row, column, or box).

I think the W-wing is the way to go here, as Gianni pointed out.

immpy · Joined: 06 May 2017 Posts: 571

Thanks for the link to Keith's article Marty, that is some good reading there.
More for our brains to try and wrap around.

Marty R. · Posted: Tue Mar 06, 2018 1:29 pm Post subject:

Just my personal practice; when the potential UR has too many candidates to qualify as a Type X, I refer to it as a DP.

mtharp · Joined: 19 Jun 2006 Posts: 30

Marty, as per immp, thanks for the link to Keith's tutorial. Enlightening.

Showing my ignorance, I have to ask, what is DP (unless it refers to the pornographic term).

I am now reading up on W wings. Hope I do not bother you folks with too many questions.

Mike

Marty R. · Posted: Wed Mar 07, 2018 4:37 pm Post subject: