Welcome!
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, 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 last (tough!) Classic Sudoku puzzle of the year.
I have solved it in 6 steps (4 of them are related to wings)
(found with Sudoku Architect)
,-----------------------------------------------------------,
| 2468 268 468 | 389 7 3689 | 5 1 49 | 9
| 9 7 58 | 1 4 58 | 6 3 2 | 8
| 3 1 456 | 59 2 569 | 7 8 49 | 7
|---------------------+---------------------+---------------|
| 257 25 139 | 6 139 27 | 19 4 8 | 6
| 128 4 189 | 29 5 129 | 3 6 7 | 5
| 167 69 1369 | 379 8 4 | 19 2 5 | 4
|---------------------+---------------------+---------------|
| 1458 3 1489 | 2589 19 12589 | 248 7 6 | 3
| 4568 568 2 | 3578 36 3578 | 48 9 1 | 2
| 168 689 7 | 4 169 1289 | 28 5 3 | 1
'-----------------------------------------------------------'
a b c d e f g h i
Update (Dec 19, 2025)
The bivalued cells (27)f6, (29)d6, (59)d7 seem useful for a start
and indeed they are part of move 1 that eliminates (2)f6 using
interactions between the three central boxes. This gives 3 placements
and one LC elimination.
,-----------------------------------------------------------,
| 2468 268 468 | 389 7 3689 | 5 1 49 | 9
| 9 7 58 | 1 4 58 | 6 3 2 | 8
| 3 1 456 | 59 2 569 | 7 8 49 | 7
|---------------------+---------------------+---------------|
| 25 25 139 | 6 139 7 | 19 4 8 | 6
| 18 4 189 | 29 5 129 | 3 6 7 | 5
| 7 69 1369 | 39 8 4 | 19 2 5 | 4
|---------------------+---------------------+---------------|
| 1458 3 1489 | 2589 19 12589 | 248 7 6 | 3
| 4568 568 2 | 7 36 358 | 48 9 1 | 2
| 168 689 7 | 4 169 1289 | 28 5 3 | 1
'-----------------------------------------------------------'
a b c d e f g h i
These eliminations opened the road to a powerful (hidden) wing.
Update (Dec 20, 2025)
One can get -9 f1 from a fish on 9, but using one cell LESS, we get more!
ADDED: From this moves one gets [after two placements and LC]
,---------------------------------------------------------,
| 248 28 468 | 398 7 3689 | 5 1 49 | 9
| 9 7 58 | 1 4 58 | 6 3 2 | 8
| 3 1 456 | 59 2 569 | 7 8 49 | 7
|-------------------+---------------------+---------------|
| 25 25 139 | 6 139 7 | 19 4 8 | 6
| 18 4 189 | 29 5 129 | 3 6 7 | 5
| 7 6 139 | 39 8 4 | 19 2 5 | 4
|-------------------+---------------------+---------------|
| 1458 3 148 | 2589 19 12589 | 248 7 6 | 3
| 4568 85 2 | 7 36 358 | 48 9 1 | 2
| 168 9 7 | 4 16 128 | 28 5 3 | 1
'---------------------------------------------------------'
a b c d e f g h i
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 ?
ADDED: the UR move is not needed for a krakenless solution in three steps.
In my first solution path, after the UR a complex almost chain was found.
However, for a neat krakenless solution (my 2nd solution path) in three steps,
one can take as move 1 the available Y-wing, with 3 placements and 2 LC eliminations.
,--------------------------------------------------,
| 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
So, the hard part of this puzzle is the next move for this
configuration.
Update (Dec 15, 2025)
One key step for solving this puzzle is to remove (7)e5.
How to prove that (7)e5 is false ? How this target was
found ?
Update (Dec 16, 2025)
Column 3 is a weakness, as well as row 3 and boxes b8, h2.
The key ingredients of the chain use just a bit of that:
. either NP(79)d6.f6 or (6)a8
. either NT(347)g258 or (6)g8
. (6)a8 - (6)g8 [both cannot be true]
,--------------------------------------------------,
| 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 38-7 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
After that elimination we get
,------------------------------------------------------,
| 7 38 5 | 26 389 4 | 369 1 2389 | 9
| 26 9 4 | 5 378 1 | 367 378 238 | 8
| 1 38 26 | 26 79 38 | 5 47 49 | 7
|----------------+------------------+------------------|
| 34 147 8 | 79 2 379 | 139 5 6 | 6
| 5 2 9 | 1 38 6 | 347 3478 348 | 5
| 36 17 16 | 4 5 3789 | 139 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
How to finish the puzzle ?
ADDED: Look for a well-concealed M-wing !
Update (Dec 18, 2025)
The M-wing is based on the strong links: (7)g4 = (7)h4, (4)g2 = (4)g4.
The elimination of (4)h3 gives 26 placements and a LC elimination.
The resulting position has a few "winging" ways to finish
(Y-wing, or anyone of two possible XYZ-wings)
,-------------------------------------------,
| 7 38 5 | 6 389 4 | 39 1 2 | 9
| 2 9 4 | 5 37 1 | 6 78 38 | 8
| 1 38 6 | 2 79 38 | 5 47 49 | 7
|------------+--------------+---------------|
| 3 4 8 | 7 2 9 | 1 5 6 | 6
| 5 2 9 | 1 38 6 | 7 48 348 | 5
| 6 7 1 | 4 5 38 | 39 2 389 | 4
|------------+--------------+---------------|
| 4 6 7 | 9 1 2 | 8 3 5 | 3
| 8 1 2 | 3 6 5 | 4 9 7 | 2
| 9 5 3 | 8 4 7 | 2 6 1 | 1
'-------------------------------------------'
a b c d e f g h i
Created: February 12, 2024
Contact: sudo.jco.br@gmail.com