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Chicago Tribune 1/5/18

 
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mtharp



Joined: 19 Jun 2006
Posts: 30

PostPosted: Sat Jan 06, 2018 4:38 am    Post subject: Chicago Tribune 1/5/18 Reply with quote

daily sudoku graded this one as too hard. I got stuck here:
Code:

+-----------------+-----------+-----------------+
| 2     1     9   | 6 7   35  | 45   8     34   |
| 35    8     7   | 4 13  9   | 2    6     135  |
| 346   345   346 | 2 8   135 | 1579 3579  1379 |
+-----------------+-----------+-----------------+
| 1679  2     5   | 3 69  4   | 179  79    8    |
| 34679 3479  346 | 1 569 8   | 4579 34579 2    |
| 8     349   134 | 7 59  2   | 6    3459  1349 |
+-----------------+-----------+-----------------+
| 13479 34579 134 | 8 2   13  | 4579 4579  6    |
| 145   6     8   | 9 14  7   | 3    2     45   |
| 3479  3479  2   | 5 34  6   | 8    1     479  |
+-----------------+-----------+-----------------+

Play this puzzle online at the Daily Sudoku site

Any logic or pattern help appreciated.

Mike
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ZeroAssoluto



Joined: 05 Feb 2017
Posts: 285
Location: Rimini, Italy

PostPosted: Sat Jan 06, 2018 9:20 am    Post subject: Reply with quote

Hi everyone,

first step: Finned Sworfish with number 1 in r3c69,r6c39,r7c36 finned in r2c9 and -1 in r3c7

Code:

+-----------------+-----------+-----------------+
| 2     1     9   | 6 7   35  | 45   8     34   |
| 35    8     7   | 4 13  9   | 2    6     135  |
| 346   345   346 | 2 8   135 | 579  3579  1379 |
+-----------------+-----------+-----------------+
| 679   2     5   | 3 69  4   | 1    79    8    |
| 34679 3479  346 | 1 569 8   | 4579 34579 2    |
| 8     349   1   | 7 59  2   | 6    3459  349  |
+-----------------+-----------+-----------------+
| 13479 34579 34  | 8 2   13  | 4579 4579  6    |
| 145   6     8   | 9 14  7   | 3    2     45   |
| 3479  3479  2   | 5 34  6   | 8    1     479  |
+-----------------+-----------+-----------------+

Play this puzzle online at the Daily Sudoku site

then
XY-Wing 1,3,4 in r7c36,r8c5 and -4 in r8c1
XY-Wing 1,3,5 in r2c15,r8c1 and -1 in r8c5

Ciao Gianni
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immpy



Joined: 06 May 2017
Posts: 186

PostPosted: Sat Jan 06, 2018 2:09 pm    Post subject: Reply with quote

mtharp, a personal favorite of mine....the finned XY-Wing solves this one.

Notice the 135 combo 35@r1c6; 13@r2c5; and 135@r2c9

...if this were a true XY-Wing r2c9 would include ONLY 15 as candidates, but alas, that pesky fin of 3!! One could say this is an "almost" XY- Wing....
It is indeed an XY-Wing with a fin (of 3). In this case the "fin" is in a pincer cell, but it can show up in the pivot as well. Here the pivot is 13, which sees the 35 and 135 as pincers.

In all cases, either the XY-Wing is true, or the fin is true....one must go about trying to solve using either the XY-Wing as a basis, OR using the fin to fill r2c9 as a basis.....

Upon trying the XY-Wing as if true, a contradiction arose midway thru the process of mock-solving....hence, the fin (3) was next to be plugged into r2c9....and this easily solved the puzzle...one still had to use basics in spots for common line, row, box eliminations; but it became basically stte.

Thank you mtharp.....very nice scheme, and a great puzzle.....

p.s. I usually avoid swordfish, they are too hard for me to spot, and there are other ways to crack most puzzles......

immpy
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mtharp



Joined: 19 Jun 2006
Posts: 30

PostPosted: Sat Jan 06, 2018 8:47 pm    Post subject: Reply with quote

Gianni, thanks. Elegant solution. I will need to work on spotting swordfish, included the finned variety.

Mike


ZeroAssoluto wrote:
Hi everyone,

first step: Finned Sworfish with number 1 in r3c69,r6c39,r7c36 finned in r2c9 and -1 in r3c7

Code:

+-----------------+-----------+-----------------+
| 2     1     9   | 6 7   35  | 45   8     34   |
| 35    8     7   | 4 13  9   | 2    6     135  |
| 346   345   346 | 2 8   135 | 579  3579  1379 |
+-----------------+-----------+-----------------+
| 679   2     5   | 3 69  4   | 1    79    8    |
| 34679 3479  346 | 1 569 8   | 4579 34579 2    |
| 8     349   1   | 7 59  2   | 6    3459  349  |
+-----------------+-----------+-----------------+
| 13479 34579 34  | 8 2   13  | 4579 4579  6    |
| 145   6     8   | 9 14  7   | 3    2     45   |
| 3479  3479  2   | 5 34  6   | 8    1     479  |
+-----------------+-----------+-----------------+

Play this puzzle online at the Daily Sudoku site

then
XY-Wing 1,3,4 in r7c36,r8c5 and -4 in r8c1
XY-Wing 1,3,5 in r2c15,r8c1 and -1 in r8c5

Ciao Gianni
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mtharp



Joined: 19 Jun 2006
Posts: 30

PostPosted: Sat Jan 06, 2018 9:01 pm    Post subject: Reply with quote

immpy, thanks for the help. I generally prefer to recognize a pattern, but as you point out swordfish are hard to see (at least for me). Mock solving seems a bit like guessing to me, but at least the finned XY-Wing gives you a logical place to start.

Again, thanks.

Mike


immpy wrote:
mtharp, a personal favorite of mine....the finned XY-Wing solves this one.

Notice the 135 combo 35@r1c6; 13@r2c5; and 135@r2c9

...if this were a true XY-Wing r2c9 would include ONLY 15 as candidates, but alas, that pesky fin of 3!! One could say this is an "almost" XY- Wing....
It is indeed an XY-Wing with a fin (of 3). In this case the "fin" is in a pincer cell, but it can show up in the pivot as well. Here the pivot is 13, which sees the 35 and 135 as pincers.

In all cases, either the XY-Wing is true, or the fin is true....one must go about trying to solve using either the XY-Wing as a basis, OR using the fin to fill r2c9 as a basis.....

Upon trying the XY-Wing as if true, a contradiction arose midway thru the process of mock-solving....hence, the fin (3) was next to be plugged into r2c9....and this easily solved the puzzle...one still had to use basics in spots for common line, row, box eliminations; but it became basically stte.

Thank you mtharp.....very nice scheme, and a great puzzle.....

p.s. I usually avoid swordfish, they are too hard for me to spot, and there are other ways to crack most puzzles......

immpy
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA

PostPosted: Sat Jan 06, 2018 10:44 pm    Post subject: Reply with quote

Quote:
p.s. I usually avoid swordfish, they are too hard for me to spot, and there are other ways to crack most puzzles.


I play a swordfish about once per decade. Looking for them is too much work for too little return.

I've never played a Finned Swordfish, but I'm wondering if Gianni's last two cell designations could be reversed. It looks to me that r3c7 is a fin and r2c9 an elimination.
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mtharp



Joined: 19 Jun 2006
Posts: 30

PostPosted: Sun Jan 07, 2018 5:43 am    Post subject: Reply with quote

Marty, it appears it can be reversed and played either way, albeit with a different set of reasonably basic moves. With your interpretation of the fin, the solution was:

chicago 1/5/18 v2
Code:

+-------+-------+-------+
| 2 1 9 | 6 7 3 | 5 8 4 |
| 5 8 7 | 4 1 9 | 2 6 3 |
| 6 3 4 | 2 8 5 | 7 9 1 |
+-------+-------+-------+
| 9 2 5 | 3 6 4 | 1 7 8 |
| 3 7 6 | 1 9 8 | 4 5 2 |
| 8 4 1 | 7 5 2 | 6 3 9 |
+-------+-------+-------+
| 7 5 3 | 8 2 1 | 9 4 6 |
| 1 6 8 | 9 4 7 | 3 2 5 |
| 4 9 2 | 5 3 6 | 8 1 7 |
+-------+-------+-------+


which was exactly the same as Gianni's.

Thanks for your sharp eye and making me aware that the fin could be "inside" the swordfish.

Mike


Marty R. wrote:
Quote:
p.s. I usually avoid swordfish, they are too hard for me to spot, and there are other ways to crack most puzzles.


I play a swordfish about once per decade. Looking for them is too much work for too little return.

I've never played a Finned Swordfish, but I'm wondering if Gianni's last two cell designations could be reversed. It looks to me that r3c7 is a fin and r2c9 an elimination.
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ZeroAssoluto



Joined: 05 Feb 2017
Posts: 285
Location: Rimini, Italy

PostPosted: Sun Jan 07, 2018 8:47 am    Post subject: Reply with quote

Marty R. wrote:

I've never played a Finned Swordfish, but I'm wondering if Gianni's last two cell designations could be reversed. It looks to me that r3c7 is a fin and r2c9 an elimination.


Hi Marty,

for how I know it, if the Finned Swordfish is on the columns (c369 in this situation), finned is on the column and the elimination on the row.

but I could be wrong, I'm not a great expert;
this is the explanation I found on a manual or on an internet site

Ciao Gianni
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keith



Joined: 19 Sep 2005
Posts: 3340
Location: near Detroit, Michigan, USA

PostPosted: Sun Jan 07, 2018 8:59 am    Post subject: Reply with quote

I think that much of this is a waste of time. For years I have been solving Sudoku by looking for strong links in a single variable. Here is the puzzle:

Code:
+-----------------+-----------+-----------------+
| 2     1     9   | 6 7   35  | 45   8     34   |
| 35    8     7   | 4 13  9   | 2    6     135  |
| 346   345   346 | 2 8   135 | 1579 3579  1379 |
+-----------------+-----------+-----------------+
| 1679  2     5   | 3 69  4   | 179  79    8    |
| 34679 3479  346 | 1 569 8   | 4579 34579 2    |
| 8     349   134 | 7 59  2   | 6    3459  1349 |
+-----------------+-----------+-----------------+
| 13479 34579 134 | 8 2   13  | 4579 4579  6    |
| 145   6     8   | 9 14  7   | 3    2     45   |
| 3479  3479  2   | 5 34  6   | 8    1     479  |
+-----------------+-----------+-----------------+


In which cells does 1 occur as a candidate? Here they are:

Code:
+-------+-------+-------+
| . . . | . . . | . . . |
| . . . | . X . | . . X |
| . . . | . . X | X . X |
+-------+-------+-------+
| X . . | . . . | X . . |
| . . . | . . . | . . . |
| . . X | . . . | . . X |
+-------+-------+-------+
| X . X | . . X | . . . |
| X . . | . X . | . . . |
| . . . | . . . | . . . |
+-------+-------+-------+ 

Now, look at the strong links. Here are some of them:

Code:
+-------+-------+-------+
| . . . | . . . | . . . |
| . . . | . A . | . . a |
| . . . | . . X | D . X |
+-------+-------+-------+
| B . . | . . . | b . . |
| . . . | . . . | . . . |
| . . X | . . . | . . E |
+-------+-------+-------+
| X . X | . . X | . . . |
| C . . | . c . | . . . |
| . . . | . . . | . . . |
+-------+-------+-------+ 

It takes only a moment to see that a and b are pincers on 1, and that 1 can be eliminated in D and E.

What is this? I call it three strong links. Maybe it's a stacked skyscraper or multi-coloring. Maybe it's a kraken, sashimi, or finned something-fish. I don't care, but I don't have to carry this encyclopedia of possible patterns in my head.

By the way, I agree with Marty. Looking for swordfish and other fish is a waste of time. And, an X-wing is two strong links, is a skyscraper, no?

Keith
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Marty R.



Joined: 12 Feb 2006
Posts: 5770
Location: Rochester, NY, USA

PostPosted: Sun Jan 07, 2018 3:26 pm    Post subject: Reply with quote

Quote:
Marty, it appears it can be reversed and played either way, albeit with a different set of reasonably basic moves.


Mike, my comments were based on a false interpretation and assumption. I was assuming that the swordfish pattern was just in the rows but I now see that it's in both rows and columns.
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mtharp



Joined: 19 Jun 2006
Posts: 30

PostPosted: Mon Jan 08, 2018 12:50 am    Post subject: Reply with quote

Keith, your solution seems beautifully straightforward.

As a relative newcomer, I am probably asking questions that have been answered before, but how do you pick which strong links to use for eliminations?

Why not use (what I understand to be) strong links r2c5, r8c5 and r6c3, r7c3 to eliminate the 1 in r8c1.

Thanks for wading in.

Mike

keith wrote:
I think that much of this is a waste of time. For years I have been solving Sudoku by looking for strong links in a single variable. Here is the puzzle:

Code:
+-----------------+-----------+-----------------+
| 2     1     9   | 6 7   35  | 45   8     34   |
| 35    8     7   | 4 13  9   | 2    6     135  |
| 346   345   346 | 2 8   135 | 1579 3579  1379 |
+-----------------+-----------+-----------------+
| 1679  2     5   | 3 69  4   | 179  79    8    |
| 34679 3479  346 | 1 569 8   | 4579 34579 2    |
| 8     349   134 | 7 59  2   | 6    3459  1349 |
+-----------------+-----------+-----------------+
| 13479 34579 134 | 8 2   13  | 4579 4579  6    |
| 145   6     8   | 9 14  7   | 3    2     45   |
| 3479  3479  2   | 5 34  6   | 8    1     479  |
+-----------------+-----------+-----------------+


In which cells does 1 occur as a candidate? Here they are:

Code:
+-------+-------+-------+
| . . . | . . . | . . . |
| . . . | . X . | . . X |
| . . . | . . X | X . X |
+-------+-------+-------+
| X . . | . . . | X . . |
| . . . | . . . | . . . |
| . . X | . . . | . . X |
+-------+-------+-------+
| X . X | . . X | . . . |
| X . . | . X . | . . . |
| . . . | . . . | . . . |
+-------+-------+-------+ 

Now, look at the strong links. Here are some of them:

Code:
+-------+-------+-------+
| . . . | . . . | . . . |
| . . . | . A . | . . a |
| . . . | . . X | D . X |
+-------+-------+-------+
| B . . | . . . | b . . |
| . . . | . . . | . . . |
| . . X | . . . | . . E |
+-------+-------+-------+
| X . X | . . X | . . . |
| C . . | . c . | . . . |
| . . . | . . . | . . . |
+-------+-------+-------+ 

It takes only a moment to see that a and b are pincers on 1, and that 1 can be eliminated in D and E.

What is this? I call it three strong links. Maybe it's a stacked skyscraper or multi-coloring. Maybe it's a kraken, sashimi, or finned something-fish. I don't care, but I don't have to carry this encyclopedia of possible patterns in my head.

By the way, I agree with Marty. Looking for swordfish and other fish is a waste of time. And, an X-wing is two strong links, is a skyscraper, no?

Keith
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immpy



Joined: 06 May 2017
Posts: 186

PostPosted: Mon Jan 08, 2018 1:06 am    Post subject: Reply with quote

Keith's post is a priceless gem of information. Highlight it, copy it, paste it somewhere where you can read it often. It should be required reading for any and all Sudoku solvers. For me, this is going to shed a whole new light on some super hard puzzles. One great light bulb has been brightly illuminated, while about 10 other dimly lit bulbs (fish, loops, multi-coloring, Medusas, wraps, traps, etc.) can be left just as they are (dim at best).

Thank you Keith, I am heading down a whole new road enthusiastically.

immpy

p.s. After all, would I really be able to spot a finned Mutant Leviathan even it was screaming at me?? LOL
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keith



Joined: 19 Sep 2005
Posts: 3340
Location: near Detroit, Michigan, USA

PostPosted: Mon Jan 08, 2018 8:48 am    Post subject: Reply with quote

Mike and immpy,

Thank you! I am in the process of writing this down.

You can make eliminations by pairing strong links in any order. The other eliminations will not go away. Make the elimination, modify the diagram, and look for more.

A good question, which I have already answered, is how do you know which candidates to draw the strong link diagrams for?

Take a look here:

http://forum.enjoysudoku.com/finding-the-possibility-of-single-candidate-eliminations-t32748.html

and study the example. In most puzzles, after basics, the possibility of strong link eliminations will lie with only one or two candidates. You draw strong link diagrams only for those candidates.

I have also written an article on how to do basics. It is here:

http://www.dailysudoku.com/sudoku/forums/viewtopic.php?t=8180&sid=cbf58c9d0597624a2013d1249ba2a1df

The idea of "sweeping" the floors and the chimneys as a first step is very useful. It also starts to look at the big picture: What is the pattern of each unsolved candidate?

More to come.

Keith
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keith



Joined: 19 Sep 2005
Posts: 3340
Location: near Detroit, Michigan, USA

PostPosted: Mon Jan 08, 2018 9:09 am    Post subject: Reply with quote

Quote:
Why not use (what I understand to be) strong links r2c5, r8c5 and r6c3, r7c3 to eliminate the 1 in r8c1.


Mike, that is not a valid elimination. The two strong links do not line up with each other.

Take a look at Havard's classic explanation:

http://forum.enjoysudoku.com/strong-links-for-beginners-t3326.html

Unfortunately, a couple of his images have disappeared in the decade since he wrote it.

THIS should be required reading for all Sudoku solvers.

Keith
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keith



Joined: 19 Sep 2005
Posts: 3340
Location: near Detroit, Michigan, USA

PostPosted: Mon Jan 08, 2018 10:18 am    Post subject: Reply with quote

Quote:
After all, would I really be able to spot a finned Mutant Leviathan even it was screaming at me?? LOL


immpy, that is funny, but also not funny. Each n-fish (Leviathan= 7 ) in the columns/rows has a complementary 9-n fish in the rows/columns. So, a Leviathan has an accompanying X-wing that makes the same eliminations. I used to think looking for higher-order fish is a waste of time, now I think looking for any fish is a waste of time. The complement of your finned mutant Leviathan is probably a skyscraper.

Why this is not funny is that most of the discussion groups have been taken over by computer programmers, who have driven away the human solvers.

Keith

PS. Don't tell Marty. In real life he is an avid fisherman.
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keith



Joined: 19 Sep 2005
Posts: 3340
Location: near Detroit, Michigan, USA

PostPosted: Mon Jan 08, 2018 11:29 am    Post subject: Reply with quote

From the Sudoku Player's Forum:

Quote:
A Skyscraper can always be seen as a Sashimi Finned X Wing or a short (2 Strong links) X Chain


I rest my case.

Keith

Jet-lagged in Seattle
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immpy



Joined: 06 May 2017
Posts: 186

PostPosted: Wed May 09, 2018 11:58 pm    Post subject: Reply with quote

I just wanted to revisit this thread and comment that in the most recent months after this thread was originally posted, I have been able to solve some very difficult puzzles; due mostly to my ability to identify the STRONG LINKS! When it seems a puzzle offers nothing recognizable (as to the techniques I am familiar with and capable of implementing), identifying the strong links and making the eliminations they expose will eventually bring the "breakthrough" moment. Just one candidate elimination in a particular cell can make such a great difference. The strong links will reveal the proper elimination. It may not solve the puzzle in one move in and of itself in each case instance (although it is possible), but it surely can take a puzzle that I have stalled out on and allow me to resume my quest.

Thanks again to all of the community, and a special thanks to Keith. Without this source of knowledge, I would still be floundering on the difficult puzzles. Strong links are most definitely a "key" to my solving ability.

cheers...immp
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keith



Joined: 19 Sep 2005
Posts: 3340
Location: near Detroit, Michigan, USA

PostPosted: Fri May 11, 2018 11:48 pm    Post subject: Reply with quote

immpy wrote:
I just wanted to revisit this thread and comment that in the most recent months after this thread was originally posted, I have been able to solve some very difficult puzzles; due mostly to my ability to identify the STRONG LINKS! When it seems a puzzle offers nothing recognizable (as to the techniques I am familiar with and capable of implementing), identifying the strong links and making the eliminations they expose will eventually bring the "breakthrough" moment. Just one candidate elimination in a particular cell can make such a great difference. The strong links will reveal the proper elimination. It may not solve the puzzle in one move in and of itself in each case instance (although it is possible), but it surely can take a puzzle that I have stalled out on and allow me to resume my quest.

Thanks again to all of the community, and a special thanks to Keith. Without this source of knowledge, I would still be floundering on the difficult puzzles. Strong links are most definitely a "key" to my solving ability.

cheers...immp
immpy,

Thank you very much!

This idea of single-digit strong links is indeed very powerful.

Keith
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keith



Joined: 19 Sep 2005
Posts: 3340
Location: near Detroit, Michigan, USA

PostPosted: Sun May 13, 2018 12:59 am    Post subject: Reply with quote

I have, at last, written down my thoughts on Single-Digit Strong Links.

http://www.dailysudoku.com/sudoku/forums/viewtopic.php?p=39993&sid=b087e024632f261bcb9135dcfe4e77d1#39993

Thanks to immpy for the final nudge.

(That post will not show up as new, since I have edited a placeholder.)

Keith
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