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This is a place chosen to make available some ideas (on puzzles, patterns, etc)
that seemed unsuitable for posting in a Forum, but some people may find interesting.
For difficult puzzles (like the next one), 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).
I propose this pretty resilient and interesting tough 9x9 Classic Sudoku puzzle.
(found with Sudoku Architect)
,----------------------------------------------------,
| 7 358 256 | 26 389 4 | 369 1 2389 | 9
| 268 9 4 | 5 378 1 | 367 378 238 | 8
| 1 38 26 | 26 3789 378 | 5 3478 3489 | 7
|---------------+-----------------+------------------|
| 34 147 8 | 379 2 379 | 1379 5 6 | 6
| 5 2 9 | 1 378 6 | 347 3478 348 | 5
| 36 17 16 | 4 5 3789 | 1379 2 389 | 4
|---------------+-----------------+------------------|
| 49 6 7 | 39 1 2 | 8 34 5 | 3
| 248 14 125 | 38 6 35 | 34 9 7 | 2
| 89 58 3 | 789 4 579 | 2 6 1 | 1
'----------------------------------------------------'
a b c d e f g h i
Update (Dec 13, 2025)
One must decide to take (or not) the UR: UR(26)c79,d79 => +5 c9 [2 placements and basics]
,-----------------------------------------------------------,
| 7 38 5 | 26 389 4 | 369 1 2389 | 9
| 26 9 4 | 5 378 1 | 367 378 238 | 8
| 1 38 26 | 26 3789 378 | 5 3478 3489 | 7
|-------------------+-------------------+-------------------|
| 34 147 8 | 79 2 379 | 1379 5 6 | 6
| 5 2 9 | 1 378 6 | 347 3478 348 | 5
| 36 17 16 | 4 5 3789 | 1379 2 389 | 4
|-------------------+-------------------+-------------------|
| 49 6 7 | 39 1 2 | 8 34 5 | 3
| 248 14 12 | 38 6 5 | 34 9 7 | 2
| 89 5 3 | 789 4 79 | 2 6 1 | 1
'-----------------------------------------------------------'
a b c d e f g h i
The next (effective) move is hard. How to continue ?
I propose this interesting tough 9x9 Classic Sudoku puzzle.
It is solvable in three steps.
(found with Sudoku Architect)
,--------------------------------------------------------------------,
| 4789 4789 1 | 479 5 6 | 79 3 2 |
| 5679 579 3 | 279 8 27 | 4 56 1 |
| 45679 2 469 | 1 379 347 | 579 8 67 |
|----------------------+----------------------+----------------------|
| 13457 1457 48 | 4567 167 4578 | 2 9 345 |
| 34579 4579 2489 | 2457 27 24578 | 6 1 345 |
| 145 6 24 | 3 12 9 | 8 7 45 |
|----------------------+----------------------+----------------------|
| 2 19 5 | 679 3679 37 | 13 4 8 |
| 469 3 469 | 8 29 1 | 57 256 67 |
| 168 18 7 | 25 4 235 | 13 26 9 |
'--------------------------------------------------------------------'
Update (Dec 8, 2025)
There is a powerful move the reduces SE to 7.1. The idea is to look for
an almost wing in the puzzle and try to see if the spoiler
leads to the same elimination of the wing.
ADDED: search for an almost Y-wing in the top part of the puzzle.
Update (Dec 9, 2025)
,--------------------------------------------------------------------,
| 4789 4789 1 | 479 5 6 |*79 3 2 | 9
| 5679 579 3 | 279 8 27 | 4 56 1 | 8
| 45679 2 *69(4) | 1 379 347 | 57-9 8 *67 | 7
|----------------------+----------------------+----------------------|
| 13457 1457 48 | 4567 167 4578 | 2 9 345 | 6
| 34579 4579 2489 | 2457 27 24578 | 6 1 345 | 5
| 145 6 24 | 3 12 9 | 8 7 45 | 4
|----------------------+----------------------+----------------------|
| 2 19 5 | 679 3679 37 | 13 4 8 | 3
| 469 3 469 | 8 29 1 | 57 256 67 | 2
| 168 18 7 | 25 4 235 | 13 26 9 | 1
'--------------------------------------------------------------------'
a b c d e f g h i
Either we have the Y-wing (679) at the marked (*) cells above, or (4)c7,
and this second possibility also implies that (9)g7 is false (How?).
This move gives only one placement (plus LC elimination).
,-----------------------------------------------------------------,
| 478 478 1 | 47 5 6 | 9 3 2 | 9
| 5679 579 3 | 279 8 27 | 4 56 1 | 8
| 4569 2 469 | 1 39 34 | 57 8 67 | 7
|---------------------+----------------------+--------------------|
| 13457 1457 48 | 4567 167 4578 | 2 9 345 | 6
| 34579 4579 2489 | 2457 27 24578 | 6 1 345 | 5
| 145 6 24 | 3 12 9 | 8 7 45 | 4
|---------------------+----------------------+--------------------|
| 2 19 5 | 679 3679 37 | 13 4 8 | 3
| 469 3 469 | 8 29 1 | 57 256 67 | 2
| 168 18 7 | 25 4 235 | 13 26 9 | 1
'-----------------------------------------------------------------'
a b c d e f g h i
How to continue ?
Update (Dec 10, 2025)
,-----------------------------------------------------------------,
| 478 478 1 | 47 5 6 | 9 3 2 | 9
| 5679 579 3 | 279 8 27 | 4 56 1 | 8
| 4569 2 469 | 1 39 34 | 57 8 67 | 7
|---------------------+----------------------+--------------------|
| 13457 1457 48 | 4567 167 4578 | 2 9 345 | 6
| 34579 4579 2489 | 2457 27 24578 | 6 1 345 | 5
| 145 6 24 | 3 12 9 | 8 7 45 | 4
|---------------------+----------------------+--------------------|
| 2 19 5 | 679 3679 37 | 13 4 8 | 3
| 469 3 469 | 8 29 1 | 57 256 67 | 2
| 168 18 7 | 25 4 235 | 13 26 9 | 1
'-----------------------------------------------------------------'
a b c d e f g h i
Although there is a Y-wing that gets (-2)f8, it is more productive
to look for a WXYZ-wing [move 2] (can be viewed as ALS-xz move).
Update (Dec 11, 2025)
,-----------------------------------------------------------------,
| 478 478 1 | 47 5 6 | 9 3 2 | 9
| 5679 579 3 | 279 8 27 | 4 56 1 | 8
| 4569 2 469 | 1 (39) 4-3 | 57 8 67 | 7
|---------------------+----------------------+--------------------|
| 13457 1457 48 | 4567 167 4578 | 2 9 345 | 6
| 34579 4579 2489 | 2457 27 24578 | 6 1 345 | 5
| 145 6 24 | 3 12 9 | 8 7 45 | 4
|---------------------+----------------------+--------------------|
| 2 19 5 | 679 679-3 37 | 13 4 8 | 3
| 469 3 469 | 8 (29) 1 | 57 256 67 | 2
| 168 18 7 |(25) 4 (235) | 13 26 9 | 1
'-----------------------------------------------------------------'
a b c d e f g h i
WXYZ-wing (2359) e7.e2.d1.f1 => -3 f7, e3 [9 placements]
,-------------------------------------------------------,
| 48 48 1 | 7 5 6 | 9 3 2 |
| 567 57 3 | 9 8 2 | 4 56 1 |
| 569 2 69 | 1 3 4 | 57 8 67 |
|-------------------+-----------------+-----------------|
| 13457 1457 48 | 45 6 578 | 2 9 345 |
| 34579 4579 489 | 245 27 578 | 6 1 345 |
| 45 6 2 | 3 1 9 | 8 7 45 |
|-------------------+-----------------+-----------------|
| 2 19 5 | 6 79 37 | 13 4 8 |
| 469 3 469 | 8 29 1 | 57 256 67 |
| 168 18 7 | 25 4 35 | 13 26 9 |
'-------------------------------------------------------'
Now, this puzzle can be solved in one move.
Update (Dec 12, 2025)
,-------------------------------------------------------,
| 48 48 1 | 7 5 6 | 9 3 2 |
| 57-6 57 3 | 9 8 2 | 4 56 1 |
| 569 2 69 | 1 3 4 | 57 8 7-6 |
|-------------------+-----------------+-----------------|
| 13457 1457 48 | 45 6 578 | 2 9 345 |
| 34579 4579 489 | 245 27 578 | 6 1 345 |
| 45 6 2 | 3 1 9 | 8 7 45 |
|-------------------+-----------------+-----------------|
| 2 19 5 | 6 79 37 | 13 4 8 |
| 469 3 469 | 8 29 1 | 57 256 67 |
| 168 18 7 | 25 4 35 | 13 26 9 |
'-------------------------------------------------------'
a b c d e f g h i
Either (69)a7.c7 or (6)c7 => -6 i7, a8 solves the puzzle.
Created: February 12, 2024
Contact: sudo.jco.br@gmail.com