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Jun 13 VH

 
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ZeroAssoluto



Joined: 05 Feb 2017
Posts: 641
Location: Rimini, Italy

PostPosted: Sun Jun 13, 2021 8:24 am    Post subject: Jun 13 VH Reply with quote

Hi everyone,

XY-Wing 1,2,8 in r58c9,r7c8 and -8 in r46c8
or
Skyscraper with number 1 in r37c5,r38c9 and -1 in r7c78,r8c4

Ciao Gianni


Last edited by ZeroAssoluto on Sun Jun 13, 2021 9:24 am; edited 1 time in total
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TomC



Joined: 30 Oct 2020
Posts: 143
Location: Wales

PostPosted: Sun Jun 13, 2021 8:44 am    Post subject: Reply with quote

Code:

+-------------+---------------+--------------+
| 78  12 238  | 4    9   2378 | 178  6   5   |
| 578 19 3589 | 1378 6   378  | 178  2   4   |
| 4   6  28   | 5    12  278  | 9    3   178 |
+-------------+---------------+--------------+
| 1   4  289  | 2368 7   2368 | 5    89  238 |
| 6   3  258  | 9    25  1    | 4    7   28  |
| 58  29 7    | 238  4   2358 | 1238 189 6   |
+-------------+---------------+--------------+
| 2   5  6    | 17   13  4    | 1378 18  9   |
| 3   7  4    | 126  8   9    | 126  5   12  |
| 9   8  1    | 267  235 2567 | 2367 4   237 |
+-------------+---------------+--------------+

Play this puzzle online at the Daily Sudoku site

r8c4 <> 1 using 1's in c59
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TomC



Joined: 30 Oct 2020
Posts: 143
Location: Wales

PostPosted: Sun Jun 13, 2021 9:47 am    Post subject: Reply with quote

Is a skyscraper the same as colouring?
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immpy



Joined: 06 May 2017
Posts: 470

PostPosted: Sun Jun 13, 2021 4:07 pm    Post subject: Reply with quote

Hello Tom. A Skyscraper is a special form of Turbot Fish and it can be seen as two Sashimi X-Wings. The description of the pattern sounds more complicated than it really is: Concentrate on one digit. Find two rows (or columns) that contain only two candidates for that digit. If two of those candidates are in the same column (or row), one of the other two candidates must be true. All candidates that see both of those cells can therefore be eliminated.

cheers...immp
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immpy



Joined: 06 May 2017
Posts: 470

PostPosted: Sun Jun 13, 2021 4:17 pm    Post subject: Reply with quote

The digit here is 1. And the columns are c5 and c9. 1 appears only twice in each of these columns. The offset ends of the pattern are what make the potential eliminations, in this case cells r7c5 and r8c9. These act as pincers on 1, as one of these cells must equal 1 based on the pattern. All cells that can see both of these cells will be <>1. 1 can therefore be eliminated in the cells that Gianni has noted.

cheers...immp
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TomC



Joined: 30 Oct 2020
Posts: 143
Location: Wales

PostPosted: Sun Jun 13, 2021 5:57 pm    Post subject: Reply with quote

Thanks for that immpy.

The way I saw the solution was if r8c4 = 1 then r3c9 = 1 and that leads to no 1's in box 2

iechyd da
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Mogulmeister



Joined: 03 May 2007
Posts: 963

PostPosted: Mon Jun 14, 2021 10:39 am    Post subject: Reply with quote

TomC wrote:
Is a skyscraper the same as colouring?


Here's an image that might be helpful, Tom. Your eliminated candidate is in red.

Yes in this case it is also colouring. Your elimination can also be seen in all the "fish" shapes as Immp mentioned and IS also an example of colouring. It is also a skyscraper.

The critical things they all have in common are the strong links between candidate "1". In the strong link only one of two candidates can be true. The patterns are a quick shorthand for the brain to recognize.

1) Colouring

Pick two colours - here it is green and yellow and follow the strong links on 1's.. So we start with yellow at at r7c5 then green at r3c5 then yellow at r3c9 then green at r8c9. The strong links are coloured alternately representing their different polarity - we just don't know at this stage which is which. We don't know which colour is true and which is false.

You can eliminate 1 from wherever both colours point at a cell. Your elimination you identified earlier is r8c4 - which here is in red because it is seen by both the green and yellow cells. You can additionally eliminate the 1s in r7c78 which is also seen by both the green and yellow cells in r7c5 and r8c9. Some refer to these as pincer cells.

2)Skyscraper

The two strong linked towers are the 1s in c5 and c9. Although they are of different height this works because they share the same base in r3.

The skyscraper (two vertical lines) is actually "upside down". The shared "base" are the 1's in r3c59. The pincers are r7c5 and r8c9. One of those cells must be 1 so your elimination of the 1 in r8c4 takes place. As before in the colouring example you can also eliminate the 1s in r7c78.

Afterthought:

UNLIKE the colouring example you don't need for the base r3c59 to be strong linked in a skyscraper. So long as the uprights are strong linked it will always work. Try it yourself. If r7c5 is FALSE then r8c9 MUST be TRUE. if r8c9 is FALSE then r7c5 must be TRUE.

Whichever, the 1s at r8c4 and r7c78 are toast.


Last edited by Mogulmeister on Mon Jun 14, 2021 4:01 pm; edited 10 times in total
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Mogulmeister



Joined: 03 May 2007
Posts: 963

PostPosted: Mon Jun 14, 2021 10:43 am    Post subject: Reply with quote

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Mogulmeister



Joined: 03 May 2007
Posts: 963

PostPosted: Mon Jun 14, 2021 11:27 am    Post subject: Reply with quote

...and of course it is a chain as well.

(-1)r7c5=(1)r3c5-r3c9=(1)r8c9 so r8c4<>1

In English:

"It isn't 1 at r7c5 so must be 1 at r3c5 isn't 1 at r3c9 must be 1 at r8c9".

The logic always alternates between the " - "(isn't) to "=" (must be).

Usually there is a bracketed number or numbers just to the left of the cell reference to let you know what is going on. The above example is simple as we are only on digit 1. Where there is no bracketed number the chain assumes you are still on the same digit. I didn't really need the (1) quite so much above.

You will see in some of the more involved chains that there is a transition to another digit as in the example below:

(1=4)r4c2-(4=3)r8c2 etc

In English:

"Not 1 but must be 4 in r4c2 so not 4 but must be 3 in r8c2 etc"

This is for when you move from one strong linked digit to a different one.


Last edited by Mogulmeister on Mon Jun 14, 2021 2:30 pm; edited 2 times in total
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Mogulmeister



Joined: 03 May 2007
Posts: 963

PostPosted: Mon Jun 14, 2021 1:40 pm    Post subject: Reply with quote

.finally Tom you showed the destruction of all 1s in box 2 if r8c4 = 1. It also causes another major contradiction. If you plug 1 into r8c4 and follow the strong links around in the diagram it will end up putting two 1s into box 8.
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Mogulmeister



Joined: 03 May 2007
Posts: 963

PostPosted: Mon Jun 14, 2021 2:33 pm    Post subject: Reply with quote

TomC wrote:


iechyd da


Good Luck in the Euros.
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TomC



Joined: 30 Oct 2020
Posts: 143
Location: Wales

PostPosted: Mon Jun 14, 2021 6:32 pm    Post subject: Reply with quote

Thanks for all of this Mogulmeister

Very appropriate solutions following our opening match (All the 1's)

Tom
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dongrave



Joined: 06 Mar 2014
Posts: 450

PostPosted: Tue Jun 15, 2021 11:20 am    Post subject: Reply with quote

Excellent job explaining the Eureka expressions in English mogulmeister! I had a tough time learning Eureka until one day I began thinking of them in the English terms that you explained.
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Mogulmeister



Joined: 03 May 2007
Posts: 963

PostPosted: Tue Jun 15, 2021 4:48 pm    Post subject: Thanks Reply with quote

Thanks Don.

You just explained exactly my own Eureka moment.
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