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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, 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)
(Guidelines for using this site here)
(useful to look up for the meaning of some unfamiliar concept)
The closure for SE = 7.3 Classic Sudoku puzzles follows (to be solved manually).
(found with Sudoku Architect)
.-----------------------.
| . 4 . | 1 . 5 | . . . |
| 7 . . | 4 . . | . . 6 |
| . 6 . | . . . | . . 9 |
|-------+-------+-------|
| 2 . 3 | . . . | . . . |
| . . . | 3 . 8 | . . . |
| 9 8 . | . . . | 6 . . |
|-------+-------+-------|
| . . 1 | . 8 . | . 7 5 |
| . . . | 5 . . | 3 . 2 |
| 5 . . | . . 2 | 9 . . |
'-----------------------'
Update 1 (Apr 8, 2026)
After basics,
.-------------------------------------------------------.
| 38 4 9 | 1 6 5 | 278 238 378 |
| 7 135 258 | 4 239 39 | 158 1358 6 |
| 13 6 25 | 8 237 37 | 15 4 9 |
|----------------+-------------------+------------------|
| 2 157 3 | 679 14579 14679 | 1578 1589 1478 |
| 146 157 4567 | 3 14579 8 | 1257 1259 147 |
| 9 8 457 | 2 1457 147 | 6 135 1347 |
|----------------+-------------------+------------------|
| 36 2 1 | 69 8 369 | 4 7 5 |
| 468 9 4678 | 5 147 1467 | 3 168 2 |
| 5 37 4678 | 67 1347 2 | 9 168 18 |
'-------------------------------------------------------'
How to get a good start ?
Update 2 (Apr 9, 2026)
A good move 1 is to take the available Y-wing that gives 7 placements and lc elimination.
We get
.--------------------------------------------------.
| 38 4 9 | 1 6 5 | 278 238 378 |
| 7 15 258 | 4 23 9 | 158 1358 6 |
| 13 6 25 | 8 23 7 | 15 4 9 |
|--------------+----------------+------------------|
| 2 157 3 | 67 14579 146 | 1578 1589 1478 |
| 14 157 6 | 3 14579 8 | 1257 1259 147 |
| 9 8 45 | 2 1457 14 | 6 135 1347 |
|--------------+----------------+------------------|
| 6 2 1 | 9 8 3 | 4 7 5 |
| 48 9 478 | 5 147 146 | 3 168 2 |
| 5 3 478 | 67 147 2 | 9 168 18 |
'--------------------------------------------------'
Now, it is not so simple to see how to proceed.
I propose the following tough Classic Sudoku puzzle (to be solved manually).
(found with Sudoku Architect)
,-----------------------,
| . . 7 | . 9 . | 6 . . |
| 4 . . | . . 2 | . 3 . |
| . . 3 | . 1 . | . . 5 |
|-------+-------+-------|
| . . 2 | . 6 . | . 8 . |
| . 8 5 | . . . | . . . |
| 1 . . | . . . | . . . |
|-------+-------+-------|
| 5 6 . | . 2 . | . . 8 |
| . . . | 1 5 . | . . . |
| . 2 . | 3 . . | . 1 4 |
'-----------------------'
Update 1 (Apr 4, 2026)
After basics
,--------------------------------------------------------------------,
9 | 28 15 7 | 458 9 3 | 6 24 12 |
8 | 4 15 68 | 578-6 78 2 | 1789 3 179 |
7 | 268 9 3 | 4678 1 478 | 2478 247 5 |
|----------------------+----------------------+----------------------|
6 | 379 347 2 | 79 6 5 | 1479 8 1379 |
5 | 3679 8 5 | 279 34 1 | 2479 24679 23679 |
4 | 1 347 469 | 2789 34 789 | 2479 5 23679 |
|----------------------+----------------------+----------------------|
3 | 5 6 1 | 479 2 479 | 3 79 8 |
2 | 378 347 48 | 1 5 789 | 279 2679 2679 |
1 | 789 2 89 | 3 78 6 | 5 1 4 |
'--------------------------------------------------------------------'
a b c d e f g h i
The pattern of 6 is very revealing for a good move 1: -6 d8 [& 5 placements and NP]
Update 2 (Apr 5, 2026)
So, the elimination of move 1 comes from a M3-wing pattern. We get this key position
,------------------------------------------------------,
9 | 28 15 7 | 458 9 3 | 6 24 12 |
8 | 4 15 6 | 578 78 2 | 1789 3 179 |
7 | 28 9 3 | 6 1 478 | 2478 247 5 |
|-----------------+-----------------+------------------|
6 | 379 347 2 | 79 6 5 | 1479 8 1379 |
5 | 6 8 5 | 279 34 1 | 2479 2479 2379 |
4 | 1 347 49 | 2789 34 789 | 2479 5 6 |
|-----------------+-----------------+------------------|
3 | 5 6 1 | 479 2 479 | 3 79 8 |
2 | 37 347 48 | 1 5 789 | 279 6 279 |
1 | 79 2 89 | 3 78 6 | 5 1 4 |
'------------------------------------------------------'
a b c d e f g h i
How to proceed ?
Update 3 (Apr 6, 2026)
,------------------------------------------------------,
9 | 28 15 7 | 458 9 3 | 6 24 12 |
8 | 4 15 6 | 58-7 78 2 | 1789 3 179 |
7 | 28 9 3 | 6 1 478 | 2478 247 5 |
|-----------------+-----------------+------------------|
6 | 379 347 2 | 79 6 5 | 1479 8 1379 |
5 | 6 8 5 | 29-7 34 1 | 2479 2479 2379 |
4 | 1 347 49 | 289-7 34 789 | 2479 5 6 |
|-----------------+-----------------+------------------|
3 | 5 6 1 | 49-7 2 479 | 3 79 8 |
2 | 37 347 48 | 1 5 789 | 279 6 279 |
1 | 79 2 89 | 3 78 6 | 5 1 4 |
'------------------------------------------------------'
a b c d e f g h i
Moves 2,3 for this position came naturally after noticing
another M-wing that eliminates (7)d3. Continuing that chain
we get -7 d4, -7 d5. A related chain gives: -7 d8. So, all
false sevens are eliminated and we get 5 placements and
basic eliminations. We get this:
,-----------------------------------------------------------,
| 28 15 7 | 458 9 3 | 6 24 12 |
| 4 15 6 | 58 78 2 | 1789 3 179 |
| 28 9 3 | 6 1 478 | 278 247 5 |
|-------------------+-------------------+-------------------|
| 39 34 2 | 7 6 5 | 149 8 19 |
| 6 8 5 | 29 4 1 | 279 279 3 |
| 1 7 49 | 289 3 89 | 249 5 6 |
|-------------------+-------------------+-------------------|
| 5 6 1 | 49 2 479 | 3 79 8 |
| 37 34 48 | 1 5 789 | 279 6 279 |
| 79 2 89 | 3 78 6 | 5 1 4 |
'-----------------------------------------------------------'
The puzzle is easily solved in one more (easy) move.
Update 4 (Apr 7, 2026)
So, one option for the final move has the Y-wing pattern.
Regarding the key moves 2,3:
(.) => -7 d3
[.] => -7 d4, -7 d5
{.} => -7 d8
,------------------------------------------------------------,
9 | 28 {1}5 7 --> | 458 9 -- 3 --> | 6 {24} {12} |
8 | 4 {15} 6 --- |{5}8-7 {78} 2 | 1789 3 179 |
7 | 28 9 3 | 6 1 ^ 478 | 2478 {247} 5 |
|-------------------+----------|----------+---------|--------|
6 |(9)37 347-- 2 --- |(79) 6 | 5 | 1479 8 | 1379 |
5 | 6 | 8 5 |[2]9-7 34| 1 | 2479 2479 2379 |
4 | 1 | 347 49 |[28]9-7 34| [8]79 | 2479 5 | 6 |
|---|---------------+----------|--^-------+---------v--------|
3 | 5 v 6 1 | 49-7 2 | | 479 | 3 {79} 8 |
2 | 37 347 48 | 1 5 | {[8]9}7 |{9}27 6 {9}27 |
1 |(79) --2--> 89 | 3 (7{8}) 6 | 5 1 4 |
'------------------------------------------------------------'
a b c d e f g h i
Created: February 12, 2024
Contact: sudo.jco.br@gmail.com