## Help with expert Sudoku

Post the puzzle or solving technique that's causing you trouble and someone will help

### Help with expert Sudoku

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.

LilaQ
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LilaQ

Posts: 6
Joined: 27 April 2015

### Re: Help with expert Sudoku

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

Posts: 3309
Joined: 03 June 2012