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Nov 03 VH

 
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ZeroAssoluto



Joined: 05 Feb 2017
Posts: 868
Location: Rimini, Italy

PostPosted: Wed Nov 03, 2021 10:14 am    Post subject: Nov 03 VH Reply with quote

Hi everyone,

Code:

+------------+-----------------+---------------+
| 6   3  49  | 2789 5    1     | 48   2489 78  |
| 49  8  1   | 279  6    249   | 345  2349 57  |
| 5   7  2   | 3    49   489   | 1    489  6   |
+------------+-----------------+---------------+
| 7   46 5   | 19   139  39    | 468  468  2   |
| 2   9  3   | 4    8    6     | 7    5    1   |
| 1   46 8   | 25   7    25    | 9    46   3   |
+------------+-----------------+---------------+
| 8   1  46  | 59   34   7     | 2    36   59  |
| 349 2  469 | 1589 1349 34589 | 3568 7    589 |
| 39  5  7   | 6    2    389   | 38   1    4   |
+------------+-----------------+---------------+

Play this puzzle online at the Daily Sudoku site

First step X-Wing with number 8 in r18c49 and -8 in r1c78,r8c67

Code:

+----------+----------------+-------------+
| 6  3  9  | 78   5    1    | 4   2   78  |
| 4  8  1  | 279  6    29   | 35  39  57  |
| 5  7  2  | 3    49   489  | 1   89  6   |
+----------+----------------+-------------+
| 7  46 5  | 19   139  39   | 68  468 2   |
| 2  9  3  | 4    8    6    | 7   5   1   |
| 1  46 8  | 25   7    25   | 9   46  3   |
+----------+----------------+-------------+
| 8  1  46 | 59   34   7    | 2   36  59  |
| 39 2  46 | 1589 1349 3459 | 356 7   589 |
| 39 5  7  | 6    2    389  | 38  1   4   |
+----------+----------------+-------------+

Play this puzzle online at the Daily Sudoku site

Second step XY-Wing 3,6,8 in r49c7,r7c8 and -6 in r46c8,r8c7

OR with only one step AIC in r7c8
if r7c8=6 and -6 in r46c8,r8c7
if r7c8=3 r2c7=3 r8c7=5 r4c7=6 and -6 in r46c8,r8c7

Ciao Gianni
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TomC



Joined: 30 Oct 2020
Posts: 349
Location: Wales

PostPosted: Wed Nov 03, 2021 10:46 am    Post subject: Reply with quote

From the first grid position

UR <46> in r46c28 means r4c8 must be 8

or if r4c7= 8, r9c7=3, r7c8=6 gives no 6's in box 6
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Mogulmeister



Joined: 03 May 2007
Posts: 1151

PostPosted: Wed Nov 03, 2021 12:07 pm    Post subject: Reply with quote

TomC wrote:


or if r4c7= 8, r9c7=3, r7c8=6 gives no 6's in box 6


I believe this contradiction works after basics and not just after the first position, Tom.

You would, after basics, have two "not 6" conditions in r4c7 -> 8 or 4. You dealt with 8 above but what if you chose 4 ?

Well r4c7 = 4, r1c7 = 8, r9c7 = 3 and r7c8 = 6 which also gives no 6's in box 6.

This contradiction could be generally expressed as:

(6=48)r14c7-(8=3)r9c7-(3=6)r7c8-(6=48)r46c8-(48=6)r4c7

Take 6 out of r4c7 and the loop insists it must be 6 - which it is.
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TomC



Joined: 30 Oct 2020
Posts: 349
Location: Wales

PostPosted: Wed Nov 03, 2021 12:40 pm    Post subject: Reply with quote

The way I saw it was if r4c7 can't be 8 then r4c8 must be 8
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Mogulmeister



Joined: 03 May 2007
Posts: 1151

PostPosted: Wed Nov 03, 2021 12:59 pm    Post subject: Reply with quote

Yes the flip side of r4c7 being 6 is that it is also not 8 so when you plug 8 into r4c7 you show it can't be there in the first place. Put it into the loop which evicts it.

(46=8)r4c7-(8=3)r9c7-(3=6)r7c8-(46=8)r46c8-(8)r4c7 ==> r4c8 = 8
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dongrave



Joined: 06 Mar 2014
Posts: 563

PostPosted: Wed Nov 03, 2021 3:04 pm    Post subject: Reply with quote

I used the 8 X -Wing as the 1st step and then the 356 XYZ-Wing with pivot at r8c7 (-3r9c7) as the 2nd step.
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Mogulmeister



Joined: 03 May 2007
Posts: 1151

PostPosted: Thu Nov 04, 2021 1:50 pm    Post subject: Re: Nov 03 VH Reply with quote

ZeroAssoluto wrote:


Second step XY-Wing 3,6,8 in r49c7,r7c8 and -6 in r46c8,r8c7



You can have this as a slightly modified first step after basics where it is an Almost XY wing

468-38-36 there is a 4 in r4c7

If it is FALSE then we have the XY-wing as above.
If it is TRUE then (68=4)r4c7-(4=8)r1c7-(8=3)r9c7-(3=6)r7c8 and -6 in r46c8,r8c7 also.
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