The New Sudoku Players' Forum

[francesandsmoke]

Hi, I'm relatively new but have been working very hard at sudoku as of late. this is one from the Sunday paper. This is what you're given:

7*3 / 5** / **9
*** / *7* / 45*
*** / *6* / *8*

*** / **7 / 8*5
*1* / *** / *6*
5*8 / 4** / *7*

*7* / *9*/ 5**
*25/ *1* / ***
9** / **3 / 1*7

After using basic techniques, I get:

7*3 / 54* / **9
*** / *7* / 45*
*5* / *6* / 78*

*** / **7 / 8*5
*17 / 985 / 36*
5*8 / 4** / *7*

*7* / *9* / 5**
*25 / 71* / ***
9** / *53 / 1*7

Can anyone advise me what techniques I should be using (other than guessing). Thanks!

[udosuk]

You need something as advanced as Turbot fish to make progress:

Code: Select all

 *--------------------------------------------------------------------*
 | 7      68     3      | 5      4     *128    | 26    *12     9      |
 | 1268   689    1269   | 1238   7      1289   | 4      5      1236   |
 | 124    5      1249   | 123    6      129    | 7      8      123    |
 |----------------------+----------------------+----------------------|
 | 2346   3469   2469   | 1236   23     7      | 8     -1249   5      |
 | 24     1      7      | 9      8      5      | 3      6      24     |
 | 5      369    8      | 4      23    *126    | 29     7     *12     |
 |----------------------+----------------------+----------------------|
 | 13468  7      146    | 268    9      2468   | 5      234    23468  |
 | 3468   2      5      | 7      1      468    | 69     349    3468   |
 | 9      468    46     | 268    5      3      | 1      24     7      |
 *--------------------------------------------------------------------*

Either r1c6 or r1c8 must contain the 1 on r1.
Either r6c6 or r6c9 must contain the 1 on r6.
If r4c8=1, then we must have both r1c6 and r6c6 be 1, thus having 2 1s on c6.
Therefore r4c8<>1.


It seems you need to apply this technique twice more... But I'll leave them to you...
(It'll be much more enjoyable working them out yourself! )

[francesandsmoke]

Thanks Udosuk, I never heard of turbot fish before but I'm definitely going to do some research! Is it normal for a community newspaper (mine is the Bellingham Herald) to publish such hard Sudoku puzzles? Anyway, I'm excited to learn about this strategy.

[udosuk]

It's surely not normal difficulty for a newspaper puzzle... But I think a good newspaper should have some variety on their puzzles, from the trivial for the beginners to more difficult ones for the enthusiasts...

And it's not like this one is impossible to solve using pen and paper (without candidates)... Just try to spot 2 different rows/columns with the same candidate locked in 2 cells... If both of these 2 cells are aligned then you find yourself an x-wing... If only one pair is aligned then you look for cells that "see" the other pair simulaneously (turbot fish), and make the mental elimination... Often this will lead to immediate hidden singles (just like this case)...

[ravel]

Thanks Udosuk, I never heard of turbot fish before but I'm definitely going to do some research!

Try this thread, very good explanation for turbot fish&co.

[mikosiko]

I'm really new to sodoku (just 1 week playing)... no "special programs" here... I found your problem very interesting... after few minutes looking for some keys... I found the 2 numbers that help me to solve the game very quickly.... look for the missing numbers in row 1, col 5 and then row 5 col 9 (after solving the obvious of course)... after those numbers the game just flow with simple logic.

Miko

[ravel]

r1c5 is resolved, but what do you mean with r5c9 ?

[mikosiko]

my mistake.... my key was r6c9... the previous analysis give you just one number there <> 1

[Len]

I'm able to get to this point but where to go from here?

****7 **68 ***3 / **5 *4 *28 / 26 **1 ***9
*1268 *689 1269 / *38 *7 189 / *4 **5 **36
**124 ***5 *149 / *23 *6 *19 / *7 **8 **23

**246 3469 2469 / **1 23 **7 / *8 249 ***5
***24 ***1 ***7 / **9 *8 **5 / *3 **6 **24
****5 **39 ***8 / **4 23 **6 / 29 **7 ***1

13468 ***7 *146 / 268 *9 248 / *5 234 2468
*3468 ***2 ***5 / **7 *1 *48 / 69 349 *468
****9 *468 **46 / 268 *5 **3 / *1 *24 ***7

[udosuk]

One of r1c26 must be 8.
One of r9c24 must be 8.
What happens if r2c4=8?

[Len]

r1c2=8 leads to an invalid row 9 on my homegrown software. r1c6=8 cracks it. Thanks.
Note - checking the 23 pairs in block 5 gives a direct solution for r4c5=3.

[QBasicMac]

Is it normal for a community newspaper (mine is the Bellingham Herald) to publish such hard Sudoku puzzles?

My question. Is it normal to take a symmetrical and easy puzzle as

Code: Select all

7-3  5--  --9
---  -7-  45-
--4  -6-  -8-

-9-  --7  8-5
-1-  ---  -6-
5-8  4--  -7-

-7-  -9-  5--
-25  -1-  ---
9--  --3  1-7


and simply remove some clues so you can call it hard? I bet they are lucky it still had one solution.

Or maybe the puzzle I imagine originally existed was what they indended to publish, but they messed up.

Mac