The New Sudoku Players' Forum

[smurfdaddy]

Don't know why I even try these, requires some advanced solving technique I don't know yet, I keep fooling around with the cells in rows 8 and 9 but I'm not getting anywhere. Help would be most appreciated. It was so easy until the end.

56 8 59 2 1 3 4 7 69
236 236 29 4 5 7 1 8 69
1 7 4 9 6 8 3 5 2

35 35 7 8 2 4 9 6 1
8 9 1 5 3 6 7 2 4
4 2 6 7 9 1 8 3 5

9 4 3 6 7 2 5 1 8
7 56 25 1 8 9 26 4 3
26 1 8 3 4 5 26 9 7

[udosuk]

Code: Select all

 *-----------*
 |.8.|213|47.|
 |...|457|18.|
 |174|968|352|
 |---+---+---|
 |..7|824|961|
 |891|536|724|
 |426|791|835|
 |---+---+---|
 |943|672|518|
 |7..|189|.43|
 |.18|345|.97|
 *-----------*

 *--------------------------------------------------*
 | 56   8    59   | 2    1    3    | 4    7    69   |
 | 236  236  29   | 4    5    7    | 1    8    69   |
 | 1    7    4    | 9    6    8    | 3    5    2    |
 |----------------+----------------+----------------|
 | 35   35   7    | 8    2    4    | 9    6    1    |
 | 8    9    1    | 5    3    6    | 7    2    4    |
 | 4    2    6    | 7    9    1    | 8    3    5    |
 |----------------+----------------+----------------|
 | 9    4    3    | 6    7    2    | 5    1    8    |
 | 7    56   25   | 1    8    9    | 26   4    3    |
 | 26   1    8    | 3    4    5    | 26   9    7    |
 *--------------------------------------------------*

r2c2 cannot be 2 (r6c2=2)... Then look for an XY-wing involving r1c1, r2c2, r4c1, r4c2 (if r4c2 is a certain candidate, r1c1 can't have any possible candidates, so one candidate can be eliminated from r4c2)...

[daj95376]

First off, you have a Naked Single in [r6c2]=2. After you set it, there are two obvious ways to solve this puzzle.

Code: Select all

 *-----------*
 |.8.|213|47.|
 |...|457|18.|
 |174|968|352|
 |---+---+---|
 |..7|824|961|
 |891|536|724|
 |426|791|835|
 |---+---+---|
 |943|672|518|
 |7..|189|.43|
 |.18|345|.97|
 *-----------*

 *--------------------------------------------------*
 | 56   8    59   | 2    1    3    | 4    7    69   |
 | 236  36   29   | 4    5    7    | 1    8    69   |
 | 1    7    4    | 9    6    8    | 3    5    2    |
 |----------------+----------------+----------------|
 | 35   35   7    | 8    2    4    | 9    6    1    |
 | 8    9    1    | 5    3    6    | 7    2    4    |
 | 4    2    6    | 7    9    1    | 8    3    5    |
 |----------------+----------------+----------------|
 | 9    4    3    | 6    7    2    | 5    1    8    |
 | 7    56   25   | 1    8    9    | 26   4    3    |
 | 26   1    8    | 3    4    5    | 26   9    7    |
 *--------------------------------------------------*


The first is an XY-Wing from [r1c2]=56 to [r2c2]=36 and [r1c4]=35 to get [r4c2]<>3. The second is to notice that it's a BUG+1 -- all the cells have 2 candidates except [r2c1]=236 (the +1 cell). If you take each candidate in [r2c1] and see how often each occurs in [r2c1]'s row+column+box, then the candidate with the largest count will be the cell value. In this case, <6> has the highest count.

[smurfdaddy]

Thank-you. I can see these things after they are pointed out. Is there some web site or source (other than Angus which I don't completely follow on XY wings, swordfish, etc. etc.) which would tell me what to look for in these situations. I can do most "very hard" puzzles with my bag of tricks with some satisfaction but want to also move on to new challenges beyond the daily newspaper posts.

[Sped]

Code: Select all

 *--------------------------------------------------*
 | 56   8    59   | 2    1    3    | 4    7    69   |
 | 236  36   29   | 4    5    7    | 1    8    69   |
 | 1    7    4    | 9    6    8    | 3    5    2    |
 |----------------+----------------+----------------|
 | 35   35   7    | 8    2    4    | 9    6    1    |
 | 8    9    1    | 5    3    6    | 7    2    4    |
 | 4    2    6    | 7    9    1    | 8    3    5    |
 |----------------+----------------+----------------|
 | 9    4    3    | 6    7    2    | 5    1    8    |
 | 7    56   25   | 1    8    9    | 26   4    3    |
 | 26   1    8    | 3    4    5    | 26   9    7    |
 *--------------------------------------------------*


It's a classic BUG+1. Set r2c1=6 to avoid the BUG. It's all singles after that.

[smurfdaddy]

Thats great, what does BUG stand for and logically why should this rule work. I'm fascinated by the never ending list of new techniques I encounter doing Sudoku.

[Sped]

BUG is defined here:

http://forum.enjoysudoku.com/viewtopic.php?t=2352

A recent discussion about solving a similar grid with BUG is here:

http://forum.enjoysudoku.com/viewtopic.php?t=4509

Information on most advanced solving techniques can be found here:

http://forum.enjoysudoku.com/viewtopic.php?t=3315

[ab]

Thank-you. I can see these things after they are pointed out. Is there some web site or source (other than Angus which I don't completely follow on XY wings, swordfish, etc. etc.) which would tell me what to look for in these situations. I can do most "very hard" puzzles with my bag of tricks with some satisfaction but want to also move on to new challenges beyond the daily newspaper posts.

You might want to try using my software
http://uk.geocities.com/aidan_001/software.html
enter your puzzle using the editor, then step through the solution to the point where you get stuck. Then switching techniques on or off highlights cells where the techniques can be applied.