Help with expert Sudoku

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Help with expert Sudoku

Postby LilaQ » Fri Oct 13, 2017 8:44 pm

Hi guys,

can anyone give me a hint on how to proceed here? I have looked for all the simple steps, and also Swordfishes and X-Wings.

Thanks in advance!!

LilaQ
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LilaQ
 
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Re: Help with expert Sudoku

Postby Leren » Sat Oct 14, 2017 8:56 am

For the benefit of other solvers here is the puzzle solved cell status in line format : 7...69...968.....753....19684.69...339.47168...68..9..6.9....5.48....369..39.64..

To keep it simple I can solve this puzzle in two moves. The first one is an obvious XWing in 7's r69 c28 as shown in the following diagram. It removes 7 from r4c8 & r7c2.

Code: Select all
*---------------------------------------------------*
| 7   12   124  | 235  6     9     | 258  234  2458 |
| 9   6    8    | 1235 12345 2345  | 25   234  7    |
| 5   3    24   | 27   248   2478  | 1    9    6    |
|---------------+------------------+----------------|
| 8   4    1257 | 6    9     25    | 257  12-7 3    |
| 3   9    25   | 4    7     1     | 6    8    25   |
| 12 *1257 6    | 8    235   235   | 9   *1247 1245 |
|---------------+------------------+----------------|
| 6   12-7 9    | 1237 12348 23478 | 278  5    128  |
| 4   8    1257 | 1257 125   257   | 3    6    9    |
| 12 *1257 3    | 9    1258  6     | 4   *127  128  |
*---------------------------------------------------*

This is followed by some basic moves : Naked pair (12) in r17c2, another naked pair (12) in r7c2 & r9c1, pointing pair claiming 1's in r8c45, pointing triple claiming in r8c456. The resulting eliminations bring you to here :

Code: Select all
*--------------------------------------------------*
| 7   12  124  | 235  6     9    | 258  234   2458 |
| 9   6   8    | 1235 12345 2345 | 25   234   7    |
| 5   3   24   | 27   248   2478 | 1    9     6    |
|--------------+-----------------+-----------------|
| 8   4  d1257 | 6    9     25   | 257 c12    3    |
| 3   9  a25   | 4    7     1    | 6    8    b25   |
|e1-2 57  6    | 8    235   235  | 9    1247  1245 |
|--------------+-----------------+-----------------|
| 6   12  9    | 37   348   3478 | 278  5     128  |
| 4   8   57   | 1257 125   257  | 3    6     9    |
| 12  57  3    | 9    58    6    | 4    127   128  |
*--------------------------------------------------*

There is an Alternating Inference Chain (AIC) in cells a-b-c-d-e for which the eureka notation is (2) r5c3 = r5c9 - (2=1) r4c8 - r4c3 = (1) r6c1

Basically this says that if cell a is not 2 then cell e is 1. Obviously if cell a is 2 then cell e is not 2. So no matter what value cell a has, cell e is not 2, so you can remove 2 from cell e = r6c1.

The puzzle solves in singles from there. I could have used some shorter chains but there would have been more moves to document.

If you want an introduction to AIC's you can read about them here.

Leren
Leren
 
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