Stuck seemingly having to guess

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Stuck seemingly having to guess

Postby User16 » Wed Dec 17, 2014 11:56 am

Hello. I have looked at different techniques but cannot pare this puzzle down further. An online solver (sudoku-solver.com) told me there were no logical solutions - i.e. trial and error. I am hoping that is not the case, and that it is just a guessing game.

It is from the ticbits Sudoku app, expert level.

Many thanks.

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Re: Stuck seemingly having to guess

Postby JasonLion » Wed Dec 17, 2014 11:27 pm

There is a remote pair on 89 (a chain of cells containing pencil marks for 8 and 9) that allows further progress.

You can certainly solve this puzzle without guessing/trial and error. However, it will take a couple of techniques at least as difficult as remote pair (or one quite a bit more advanced technique).
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Re: Stuck seemingly having to guess

Postby JC Van Hay » Wed Dec 17, 2014 11:46 pm

There are only 2 solutions for the digit 8; stte
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Re: Stuck seemingly having to guess

Postby denis_berthier » Thu Dec 18, 2014 3:57 am

User16 wrote:Hello. I have looked at different techniques but cannot pare this puzzle down further. An online solver (sudoku-solver.com) told me there were no logical solutions - i.e. trial and error. I am hoping that is not the case, and that it is just a guessing game.


Using only easy techniques (biv-chain is the super-symmetric view of Nice Loop or AIC):
Code: Select all
finned-x-wing-in-columns: n8{c9 c4}{r3 r4} ==> r4c6 ≠ 8, r4c5 ≠ 8
biv-chain[2]: r9n8{c5 c1} - c2n8{r8 r5} ==> r5c5 ≠ 8
biv-chain[2]: c8n8{r6 r2} - r3n8{c9 c4} ==> r6c4 ≠ 8
singles ==> r6c4 = 1, r5c7 = 1, r4c7 = 6
naked-triplets-in-a-row: r4{c4 c5 c6}{n3 n4 n7} ==> r4c3 ≠ 7
hidden-single-in-a-block ==> r6c3 = 7
biv-chain[3]: r2c8{n8 n9} - c6n9{r2 r7} - b8n8{r7c6 r9c5} ==> r2c5 ≠ 8
stte
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Re: Stuck seemingly having to guess

Postby Leren » Thu Dec 18, 2014 6:09 am

Here's a solution that involves no guesswork:

Code: Select all
*--------------------------------------------------------------*
| 4     9     8      | 2     1     5      | 7     3     6      |
| 7     1     6      | 34    489   389    | 2     89    5      |
| 2     3     5      |c89    67    67     | 4     1    b89     |
|--------------------+--------------------+--------------------|
| 1689  2     179    | 34    467-8 367-8  | 16    5    a89     |
| 5     68    3      |d189   689   2      | 16    7     4      |
| 1689  4     179    |d18    5     678    | 3     89    2      |
|--------------------+--------------------+--------------------|
| 3     5     4      | 7     2     89     | 89    6     1      |
| 1689  68    19     | 5     3     4      | 89    2     7      |
| 89    7     2      | 6     89    1      | 5     4     3      |
*--------------------------------------------------------------*

Follow the cells marked abcd and you see that at least 1 of r4c9 and r56c4 must be 8 so 8 can be removed from r4c56. This move is called a Grouped Skyscraper.

Code: Select all
*--------------------------------------------------------------*
| 4     9     8      | 2     1     5      | 7     3     6      |
| 7     1     6      | 34    489   389    | 2    b89    5      |
| 2     3     5      |d89    67    67     | 4     1    c89     |
|--------------------+--------------------+--------------------|
| 1689  2     179    | 34    467   367    | 16    5     89     |
| 5     68    3      | 189   689   2      | 16    7     4      |
| 1689  4     179    | 1-8   5     678    | 3    a89    2      |
|--------------------+--------------------+--------------------|
| 3     5     4      | 7     2     89     | 89    6     1      |
| 1689  68    19     | 5     3     4      | 89    2     7      |
| 89    7     2      | 6     89    1      | 5     4     3      |
*--------------------------------------------------------------*

Follow the cells marked abcd and you see that at least 1 of r6c8 and r3c4 must be 8 so 8 can be removed from r6c4. This move is called a 2 Stringed Kite.

Code: Select all
*--------------------------------------------------------------*
| 4     9     8      | 2     1     5      | 7     3     6      |
| 7     1     6      | 34    489   389    | 2     89    5      |
| 2     3     5      | 89    67    67     | 4     1     89     |
|--------------------+--------------------+--------------------|
| 189   2     19     | 34    47    37     | 6     5     89     |
| 5    a68    3      | 89    69-8  2      | 1     7     4      |
| 689   4     7      | 1     5     68     | 3     89    2      |
|--------------------+--------------------+--------------------|
| 3     5     4      | 7     2     89     | 89    6     1      |
| 1689 b68    19     | 5     3     4      | 89    2     7      |
|c89    7     2      | 6    d89    1      | 5     4     3      |
*--------------------------------------------------------------*

A bit further on follow the cells marked abcd and you see that at least 1 of r5c2 and r9c5 must be 8 so 8 can be removed from r5c5. This is another 2 Stringed Kite.

Code: Select all
*--------------------------------------------------------------*
| 4     9     8      | 2     1     5      | 7     3     6      |
| 7     1     6      | 34   b489   38-9   | 2    a89    5      |
| 2     3     5      | 89    67    67     | 4     1     89     |
|--------------------+--------------------+--------------------|
| 189   2     19     | 34    47    37     | 6     5     89     |
| 5     68    3      | 89    69    2      | 1     7     4      |
| 689   4     7      | 1     5     68     | 3     89    2      |
|--------------------+--------------------+--------------------|
| 3     5     4      | 7     2    d89     | 89    6     1      |
| 1689  68    19     | 5     3     4      | 89    2     7      |
| 89    7     2      | 6    c89    1      | 5     4     3      |
*--------------------------------------------------------------*

Next follow the cells marked abcd and you'll find that at least one of r2c8 or r7c6 must be 9, so 9 can be removed from r2c6. This move is called a W Wing.

The puzzle should then solve easily.

Leren
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Re: Stuck seemingly having to guess

Postby User16 » Thu Dec 18, 2014 7:01 am

Thank you so very much! I will study the replies and, I suspect, learn a lot!
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