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This is a place chosen to make available some ideas (on puzzles, patterns, etc)
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For difficult puzzles, possible solutions will not be shown (only hints).
For other puzzles, the solution wil be hidden so that a serious attempt can be made
without direct access to a solution.
(hopefully this will make the puzzles more interesting for newcomers)
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(useful to look up for the meaning of some unfamiliar concept)
I propose the following tough Classic Sudoku puzzle.
(found with Sudoku Explainer)
After basics
,-------------------------------------------------------,
9 | 9 34 2 | 8 135 6 | 15 145 7 |
8 | 368 1 467 | 579 359 357 | 5689 4589 2 |
7 | 5 78 67 | 179 2 4 | 1689 189 3 |
|-----------------+-----------------+-------------------|
6 | 7 2469 469 | 3 156 125 | 12589 1589 1459 |
5 | 236 2346 5 | 126 8 9 | 123 7 14 |
4 | 1 239 8 | 257 4 257 | 2359 6 59 |
|-----------------+-----------------+-------------------|
3 | 68 78 1679 | 4 1359 135 | 159 2 1569 |
2 | 26 269 169 | 159 7 8 | 4 3 1569 |
1 | 4 5 3 | 1269 169 12 | 7 19 8 |
'-------------------------------------------------------'
a b c d e f g h i Classic Sudoku
Update 1 (Oct 28, 2026)
This puzzle is solvable in 4 steps.
My first step involved an ALS W-wing.
Update 2 (Oct 30, 2026)
For step 1, we eliminate with the previous move (1)g5.g6.
We get the configuration
,-------------------------------------------------------,
9 | 9 34 2 | 8 15-3 6 | 15 145 7 |
8 | 368 1 467 | 579 359 357 | 5689 4589 2 |
7 | 5 78 67 | 179 2 4 | 1689 189 3 |
|-----------------+-----------------+-------------------|
6 | 7 2469 469 | 3 156 125 | 2589 1589 1459 |
5 | 236 2346 5 | 126 8 9 | 23 7 14 |
4 | 1 239 8 | 257 4 257 | 2359 6 59 |
|-----------------+-----------------+-------------------|
3 | 68 78 1679 | 4 1359 135 | 159 2 1569 |
2 | 26 269 169 | 159 7 8 | 4 3 1569 |
1 | 4 5 3 | 1269 169 12 | 7 19 8 |
'-------------------------------------------------------'
a b c d e f g h i
Now for step 2, we can find a chain with 4 strong links that
eliminates (3)e9, producing 7 placements and LC elimination.
We get
,-----------------------------------------------------,
9 | 9 3 2 | 8 15 6 | 15 4 7 |
8 | 68 1 4 | 579 359 357 | 5689 589 2 |
7 | 5 78 67 | 19 2 4 | 1689 189 3 |
|----------------+-----------------+------------------|
6 | 7 2469 69 | 3 156 125 | 589 1589 1459 |
5 | 3 46 5 | 16 8 9 | 2 7 14 |
4 | 1 29 8 | 257 4 257 | 3 6 59 |
|----------------+-----------------+------------------|
3 | 68 78 1679 | 4 1359 135 | 159 2 1569 |
2 | 2 69 169 | 159 7 8 | 4 3 1569 |
1 | 4 5 3 | 1269 169 12 | 7 19 8 |
'-----------------------------------------------------'
a b c d e f g h i
The next two steps solve the puzzle.
I propose the following (tough) Slitherlink puzzle.
(Source: Simon Tatham's Collection)
.---------------------------------------.
| . . . . . . . . . . |
9 | 3 2 3 1 1 2 |
| . . . . . . . . . . |
8 | 2 2 3 |
| . . . . . . . . . . |
7 | 2 1 |
| . . . . . . . . . . |
6 | 3 3 1 |
| . . . . . . . . . . |
5 | 1 2 2 |
| . . . . . . . . . . |
4 | 2 3 3 2 1 |
| . . . . . . . . . . |
3 | 3 3 2 2 |
| . . . . . . . . . . |
2 | 2 1 3 1 3 |
| . . . . . . . . . . |
1 | 1 3 |
| . . . . . . . . . . |
'---------------------------------------'
a b c d e f g h i Slitherlink
Update 1 (Oct 28, 2026)
After (23) basic moves, we get
.---------------------------------------.
| .___. . . . . . . . . |
9 | | 3 2 3 1 1 2 |
| . . . . . . . . . . |
8 | 2 2 3 |
| . . . . . .___. . . . |
7 | 2 | 1 |
| . . .___. .___. . . . . |
6 | | 3 | 3 | 1 |
| . .___. .___. . . . . . |
5 | 1 2 2 |
| . . .___. .___. . . . . |
4 | 2 | 3 | | 3 | 2 1 |
| . . . . . . . . . . |
3 | 3 | 3 | 2 2 |
| . . . .___. . . . . . |
2 | 2 1 3 1 3 |
| . . . .___. . . . . . |
1 | 1 3 | |
| . . . . .___. . . . . |
'---------------------------------------'
a b c d e f g h i Slitherlink
How to continue ?
Update 2 (Oct 30, 2026)
Looking at (3)a3 and (2)b4, we see that
we cannot have both (3t)a3, (3r)a3, so
24.(3l)a3 25.(3b)a3
.---------------------------------------.
| .___. . . . . . . . . |
9 | | 3 2 3 1 1 2 |
| . . . . . . . . . . |
8 | 2 2 3 |
| . . . . . .___. . . . |
7 | 2 | 1 |
| . . .___. .___. . . . . |
6 | | 3 | 3 | 1 |
| . .___. .___. . . . . . |
5 | 1 2 2 |
| . . .___. .___. . . . . |
4 | 2 | 3 | | 3 | 2 1 |
| . . . . . . . . . . |
3 | | 3 | 3 | 2 2 |
| .___. . .___. . . . . . |
2 | 2 1 3 1 3 |
| . . . .___. . . . . . |
1 | 1 3 | |
| . . . . .___. . . . . |
'---------------------------------------'
a b c d e f g h i SL
Also, (2b)b9 is not possible (why ?), so
26.( r)a8! 27.(2r)b9
.---------------------------------------.
| .___. . . . . . . . . |
9 | | 3 2 | 3 1 1 2 |
| . . . . . . . . . . |
8 | | 2 2 3 |
| . . . . . .___. . . . |
7 | 2 | 1 |
| . . .___. .___. . . . . |
6 | | 3 | 3 | 1 |
| . .___. .___. . . . . . |
5 | 1 2 2 |
| . . .___. .___. . . . . |
4 | 2 | 3 | | 3 | 2 1 |
| . . . . . . . . . . |
3 | | 3 | 3 | 2 2 |
| .___. . .___. . . . . . |
2 | 2 1 3 1 3 |
| . . . .___. . . . . . |
1 | 1 3 | |
| . . . . .___. . . . . |
'---------------------------------------'
a b c d e f g h i SL
How to advance from here ?
Created: February 12, 2024
Contact: sudo.jco.br@gmail.com