# My sudoku brain broke
**URL:** https://boards.straightdope.com/t/my-sudoku-brain-broke/478866
**Category:** The Game Room
**Created:** [December 27, 2008, 7:21pm UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866 "2008-12-27T19:21:37Z")
**Posts on this page:** 10
**Page:** 1
### Author:  [@Fish](https://boards.straightdope.com/u/Fish)
#### Post date: [December 27, 2008, 7:21pm UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/1 "2008-12-27T19:21:37Z")
Stuck on LA Times Sudoku page (no registration required), a 5-star puzzle from Friday December 26th. [Link.](http://games.latimes.com/index_sudoku.html?uc_feature_code=lasud&uc_full_date=20081226) Given numbers are in green; my inserts are in black.
I always seem to get to this point and get stuck. There’s obviously a technique for getting past it, but the techniques (X-wings and swordfish) are hard to grasp. What am I looking for here?
```auto
6 2 7 | _ _ _ | 9 _ 5
9 5 _ | _ 2 _ | _ 7 _
8 _ 3 | 9 5 7 | 2 _ _
- - - - - - - - - - -
4 _ _ | 5 _ _ | 7 _ 9
5 _ 9 | 7 1 4 | _ 2 _
7 _ _ | _ 3 9 | _ 5 4
- - - - - - - - - - -
3 _ 5 | _ _ _ | 6 _ 7
2 9 _ | _ 7 5 | _ 8 3
1 7 _ | _ _ _ | 5 _ 2
```
---
### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [December 27, 2008, 9:50pm UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/2 "2008-12-27T21:50:11Z")
Here’s a few clues:
R2C9 cannot be 6, because the only places that can be 6 in the second 3x3 square are in row2. That means that R2C9 can only be 1 or 8.
It is then possible to show by a chain of either/ors that if R6C7 is not 1, then R2C3 has to be 1, and vice-versa. This means that the two squares at R2C7 and R6C3 cannot be 1.
This opens up a chain of either/ors for the 1s that eliminate further candidates from being 1s. Some of the eliminated squares than have only two numbers left and can be scouted for either/or chains for other numbers, etc.
P.S., I couldn’t do this without a computer aid that shows me remaining candidates for each square.
---
### Author:  [@want2know](https://boards.straightdope.com/u/want2know)
#### Post date: [December 27, 2008, 10:24pm UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/3 "2008-12-27T22:24:20Z")
In square 3 on the top section, 8 can only go in R2, C7 or 9. That eliminates 8 from R2 in square 2 ; 8 can only go in R1.
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [December 28, 2008, 2:54pm UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/4 "2008-12-28T14:54:29Z")
Actually, the clues I gave above aren’t that helpful, since they dead-end. Here’s a much better one:
R5C2 cannot be 8, since either R7C2 is 8, or if it’s not, then by a chain of either/ors R5C9 has to be 8.
That opens up the rest of the puzzle.
---
### Author:  [@Oslo\_Ostragoth](https://boards.straightdope.com/u/Oslo_Ostragoth)
#### Post date: [December 29, 2008, 2:07am UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/5 "2008-12-29T02:07:14Z")
Since row 5 contains a 1, the only possible 1’s in column 9 are in row 2 or 3, so other 1’s can be excluded in square 3.
Lumpy - could you explain the either/or thing to me? Also, what calculator are you using? Simple Sudoku got stuck at the same place.
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### Author:  [@Oslo\_Ostragoth](https://boards.straightdope.com/u/Oslo_Ostragoth)
#### Post date: [December 29, 2008, 2:15am UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/6 "2008-12-29T02:15:21Z")
Ditto square 2 - the one’s in row 2 can be excluded and the 6’s in row 2 of square 3.
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### Author:  [@Lumpy](https://boards.straightdope.com/u/Lumpy)
#### Post date: [December 29, 2008, 2:45am UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/7 "2008-12-29T02:45:07Z")
> [@Oslo\_Ostragoth](#):
>
> Lumpy - could you explain the either/or thing to me? Also, what calculator are you using? Simple Sudoku got stuck at the same place.
I was using Simple Sudoku to display the candidates but it doesn’t solve it automatically. When I plug in Fish’s numbers I get the following results:
R7C2 is either 4 or 8. R3C2 is 1 or 4. R3C9 is 1 or 6. R5C9 is 6 or 8.
R5C2 can’t be 8 if R7C2 is 8. If it’s 4, then R3C2 has to be 1; R3C9 has to be 6; and R5C9 has to be 8. In which case R5C2 can’t be 8 anyway. I believe most Sudoku references call this type of proof “chaining”.
---
### Author:  [@Fish](https://boards.straightdope.com/u/Fish)
#### Post date: [December 29, 2008, 5:14am UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/8 "2008-12-29T05:14:53Z")
Ah, thank you **Lumpy**. I see where you’re going with that.
If only there were a simple way to see such chains without all the “if this, then that” throughout the puzzle.
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### Author:  [@Oslo\_Ostragoth](https://boards.straightdope.com/u/Oslo_Ostragoth)
#### Post date: [December 29, 2008, 7:20am UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/9 "2008-12-29T07:20:24Z")
Errrm, I’m still not getting it.
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### Author:  [@Fish](https://boards.straightdope.com/u/Fish)
#### Post date: [December 29, 2008, 7:41am UTC](https://boards.straightdope.com/t/my-sudoku-brain-broke/478866/10 "2008-12-29T07:41:01Z")
The five squares in **Lumpy** ’s example make a shape like a P, like this:
```auto
1 or 4 .... 1 or 6
R5C2 .... 6 or 8
4 or 8
```
We can test the possibilities 4 and 8 in that one box. If we choose 8, then R5C2 cannot contain 8.
If we choose 4, then you follow the possibilities around the chain and end up with 8 in the “6 or 8” box.
In both cases you have ruled out 8 in R5C2.