dailysudoku.com Forum Index dailysudoku.com
Discussion of Daily Sudoku puzzles
 
 FAQFAQ   SearchSearch   MemberlistMemberlist   UsergroupsUsergroups   RegisterRegister 
 ProfileProfile   Log in to check your private messagesLog in to check your private messages   Log inLog in 

Another one

 
Post new topic   Reply to topic    dailysudoku.com Forum Index -> Other puzzles
View previous topic :: View next topic  
Author Message
keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA
PostPosted: Sat Dec 26, 2020 8:23 pm    Post subject: Another one Reply with quote
Here is another in my "I told you so" series. Enjoy!

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


Hint: After basics, there is a "simple" one-stepper.

Keith
Back to top
View user's profile Send private message
ZeroAssoluto



Joined: 05 Feb 2017
Posts: 942
Location: Rimini, Italy
PostPosted: Sun Dec 27, 2020 8:37 am    Post subject: Reply with quote
Hi everyone,

Finned Swordfish with number 7 in r259c567, finned in r9c8 and -7 in r8c7.

Ciao Gianni
Back to top
View user's profile Send private message
dongrave



Joined: 06 Mar 2014
Posts: 568
PostPosted: Sun Dec 27, 2020 3:45 pm    Post subject: Reply with quote
Code:

+------------+--------------+--------------+
| 7   59  2  | 1   4    8   | 6   3    59  |
| 346 469 36 | 5   27   267 | 8   29   1   |
| 8   56  1  | 3   9    26  | 4   25   7   |
+------------+--------------+--------------+
| 13  78  5  | 467 178  347 | 9   167  2   |
| 136 2   36 | 9   17   5   | 137 8    4   |
| 9   78  4  | 267 1278 237 | 5   167  36  |
+------------+--------------+--------------+
| 5   34  7  | 24  6    9   | 123 14   8   |
| 2   346 8  | 47  5    1   | 37  4679 369 |
| 46  1   9  | 8   3    247 | 27  4567 56  |
+------------+--------------+--------------+

Instead of using a simple Finned Swordfish (that I couldn't find) I used a simple forcing chain. Smile

(6=4)r9c1 - r9c6 = (4-3)r4c6 = r6c6 - (3=6)r6c9 => r9c9 <> 6; stte.
Back to top
View user's profile Send private message
keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA
PostPosted: Sun Dec 27, 2020 10:57 pm    Post subject: Reply with quote
ZeroAssoluto is correct, but I have a way of coming at these things that others may find useful.

After basics:

Code:
+----------------+----------------+----------------+
| 7    59   2    | 1    4    8    | 6    3    59   |
| 346  469  36   | 5    27   267  | 8    29   1    |
| 8    56   1    | 3    9    26   | 4    25   7    |
+----------------+----------------+----------------+
| 13   78   5    | 467  178  347  | 9    167  2    |
| 136  2    36   | 9    17   5    | 137  8    4    |
| 9    78   4    | 267  1278 237  | 5    167  36   |
+----------------+----------------+----------------+
| 5    34   7    | 24   6    9    | 123  14   8    |
| 2    346  8    | 47   5    1    | 37   4679 369  |
| 46   1    9    | 8    3    247  | 27   4567 56   |
+----------------+----------------+----------------+


There is a UR and an xy-Wing (both taking out 7s) which will get you to a kite that makes the single-step elimination, -7 in R8C7.

But, look at the after basics grid above. We are interested in single-digit eliminations like you find with turbots (kites and skyscrapers) or with fish (x-Wings, swordfish, etc.)

For each unsolved digit, look at how its possibilities occur in each 3x3 box. If the possibilities lie in more than one row or column, that box is "interesting".

Look for example, at the candidate 3. In box 1 there are two candidates in row 2. Not interesting. The interesting boxes for 3 are box 4, 6, and 9. So far, so good.

Now, the critical point: If the interesting boxes are to be useful, there must be four of them arranged in a rectangle*. If not, there is no single-digit elimination.

* Full disclosure: Also, there may be six boxes arranged in a swordfish pattern, but let's keep it simple.

Get it? In this grid there is no possibility of a kite or a skyscraper or an x-Wing or ... for digit 3. Don't even bother to look.

Now look at digit 6. There are four interesting boxes: 1, 6, 7, and 9. But they do not lie in a rectangle. Move on.

The only interesting and potentially useful digit is 7. The interesting boxes are 5, 6, 8, and 9. And, they lie in a rectangle.

Now, look for single-digit strong links on 7.

Code:
+----------------+----------------+----------------+
| 7    59   2    | 1    4    8    | 6    3    59   |
| 346  469  36   | 5    27a  267A | 8    29   1    |
| 8    56   1    | 3    9    26   | 4    25   7    |
+----------------+----------------+----------------+
| 13   78   5    | 467  178  347  | 9    167  2    |
| 136  2    36   | 9    17b  5    | 137B 8    4    |
| 9    78   4    | 267  1278 237  | 5    167  36   |
+----------------+----------------+----------------+
| 5    34   7    | 24   6    9    | 123  14   8    |
| 2    346  8    | 47C  5    1    | 3-7  4679 369  |
| 46   1    9    | 8    3    247c | 27D 4567D 56   |
+----------------+----------------+----------------+


Three strong links on 7 are marked: aA, bB, and cC. The logic is:
1. At least one of a and b is not true (not 7).
2. If a is not true, A is true; if b is not true, B is true; if c is not true, C is true.
3. If A is true, c is not true.
4. C and B are thus pincers on 7. (One or both of C and B are 7.)

For extra credit, note the grouped strong link in row 9. If c is not true, one of the cells DD is true. Any cell that sees both B and the two cells DD is not 7, making the same elimination in r8c7.

A few comments:

I hope you find this useful. I do. If not, thank you anyway for reading.

I solve puzzles with pencil and paper in an armchair. I do not use software, except to format puzzles for posts like this. If you do use software in solving puzzles, all this may be not interesting to you.

I find it easy (after some practice) to scan a puzzle for interesting boxes, and thus not waste my time looking for kites and skyscrapers where they do not exist. The exercise is also useful to find basic moves I may have missed.

I apply the logic of strong links as I go. So, I have no need to remember patterns like all the species of turbots and fish, and their finned and kraken variants.

Best wishes,

Keith

Last edited by keith on Mon Dec 28, 2020 8:28 pm; edited 1 time in total
Back to top
View user's profile Send private message
immpy



Joined: 06 May 2017
Posts: 571
PostPosted: Mon Dec 28, 2020 4:56 pm    Post subject: Reply with quote
Thank you Keith, for the puzzle and the beautiful dissertation on useful strong links. More fuel for us armchair solvers to burn. I will be referring to this one often I'm sure, as it sinks in to the gray matter! Thanks again.

cheers...immp
Back to top
View user's profile Send private message
dongrave



Joined: 06 Mar 2014
Posts: 568
PostPosted: Mon Dec 28, 2020 7:28 pm    Post subject: Reply with quote
Thanks for the pointers Keith! They're much appreciated!
Back to top
View user's profile Send private message
keith



Joined: 19 Sep 2005
Posts: 3355
Location: near Detroit, Michigan, USA
PostPosted: Mon Dec 28, 2020 8:32 pm    Post subject: Reply with quote
dongrave and immpy,

Thank you very much for the positive comments. Greatly appreciated.

Keith
Back to top
View user's profile Send private message
immpy



Joined: 06 May 2017
Posts: 571
PostPosted: Sat Jan 02, 2021 10:32 pm    Post subject: Reply with quote
So useful, this search for "interesting boxes". I see it everywhere now. Previously, I simply wasn't aware of it at all, let alone how it might aid in making eliminations. Brilliant Keith!

cheers...immp
Back to top
View user's profile Send private message
Display posts from previous:   
Post new topic   Reply to topic    dailysudoku.com Forum Index -> Other puzzles All times are GMT
Page 1 of 1

 
Jump to:  
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum


Powered by phpBB © 2001, 2005 phpBB Group