#711, August 15 - August 21, 2026:
The Weekly 'Unsolvable' Sudoku Puzzle
by Richard Kröger, guest compiler
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WARNING: This is the Weekly 'Unsolvable' Sudoku, rated above 'Extreme'.
This is currently unsolvable by my solver, except perhaps with trial and error strategies.
Using the solver will not help you. (much).
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Changes March 2026
Due to the introduction of
Forcing Nets many Weekly Unsolvable puzzles can now
be solved. I have flagged all the Sudoku ones which are solvable. I am struggling to make any new ones so we'll
have to see if the Weekly can continue. The point of the Weekly was to provoke discussion and new ideas, so
that is progress. But my solution is unlikely to be optimal so alternatives are welcome.
Archive
Each week a new 'unsolvable' will be published and the previous will be accessible here from this archive section.
If you like very tough puzzles, these are for you.
Share and Enjoy
Supporter Members have access to all these puzzles going back to number 1.
Discussion...
Post an idea here...
... by: Frans Goosens
#711
With trial and error
Combination A2=123 and I9=124
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A2=1 I9=1 No solution, Fixed
Combination 2-digits cells
A6=3 Wrong, Undo calculation
A6=8 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=1 I9=2 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=1 I9=4 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=2 I9=1 No solution, Fixed
Combination 2-digits cells
B4=3 Wrong, Undo calculation
B4=6 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=2 I9=2 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=2 I9=4 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=3 I9=1 No solution, Fixed
Combination 2-digits cells
A6=1 Wrong, Undo calculation
A6=8 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=3 I9=2 Wrong, Undo calculation
All reset to initial position
************************************************************
A2=3 I9=4 ( B4=3 ) Solved,
Total solving time is : 328 sec.
Number of logical steps is : 41649
... by: Norman Powell
Opening the Same as AS, some variations, then Bowman's Bingo finds the solution.
Hidden Pair: 5 7 in D4, E5
X-Cycles: 4 in 8B
Almost Locked Sets
Set 1 C9 F9 G9 J9 with candidates 12459
Set 2 B9 B1 B4 B7 B8 with candidates 134679
With restricted Common Value 4
Rule 1: Eliminate All possibilities that can see non-Restricted Common in both ALSs
Eliminate Value 1 from Cell A9 - This is Found by AS's Forcing Network. I have ALS before FN in my chain of techniques.
Forcing Network 1st Pass:
Value 1 in Row 1 Column 7 cannot be ON so eliminate
Contradiction: More than one Value 1 ON in Row 4
Value 1 in Row 3 Column 7 cannot be ON so eliminate
Contradiction: More than one Value 1 ON in Column 9
Value 1 in Row 3 Column 9 cannot be ON so eliminate
Contradiction: More than one Value 1 ON in Box 6
This is where AS's Solver Stops, my Forcing Networks continues with:
Value 5 in Row 3 Column 9 cannot be ON so eliminate
Contradiction: More than one Value ON in Row 8 Column 5
Value 2 in Row 7 Column 7 cannot be ON so eliminate
Contradiction: No possibilities left for Row 4 Column 2
Forcing Networks 2nd Pass
Value 2 in Row 8 Column 3 cannot be ON so eliminate
Contradiction: More than one Value ON in Row 4 Column 1
Value 2 in Row 8 Column A1cannot be ON so eliminate
Contradiction: More than one Value ON in Row 4 Column 1
Bowman's Bingo
Level 0: Cell D2 is first with min poss: Try Value 3 ON and look for contradictions
Level 1: Cell A6 is first with min possibilities: Try Value 1 ON and look for contradictions
Level 2: Cell A2 is first with min possibilities: Try Value 2 ON and look for contradictions
Level 3: Cell A7 is first with min possibilities: Try Value 3 ON and look for contradictions
Switch OFF Value 3 in Cell A7 for Level 2
Contradiction: More than one Value 7 ON in Column 8
Level 3: Cell A7 is first with min possibilities: Try Value 8 ON and look for contradictions
Switch OFF Value 8 in Cell A7 for Level 2
Contradiction: No possibilities left for Row 3 Column 9
Switch OFF Value 2 in Cell A2 for Level 1
Contradiction: No possibilities left for Row 1 Column 7
Level 2: Cell A2 is first with min possibilities: Try Value 3 ON and look for contradictions
Level 3: Cell A3 is first with min possibilities: Try Value 2 ON and look for contradictions
Switch OFF Value 2 in Cell A3 for Level 2
Contradiction: No possibilities left for Row 8 Column 1
Level 3: Cell A8 is first with min poss: Try Value 2 ON and look for contradictions
Solution Found - Eliminate all Trial OFFs
... by: Dieter
#711 - initially populated cells: 22 = 27,2 %
No solver used, solver = brain.
basics: %
Strategy: numbers 4,5,6,7,8 occur most frequently, combine two cells with almost same candidates.
combinations of A9 = (1,2,5,6,7) and B9 = (1,4,6,7,9);
after basic checks 6 matrices left: 2/7, 5/7, 6/7, 7/1, 7/4 and 7/9
A9=5 & B9=7 solve with J4=7.
Optimization:
A9=5 & J4=7 solve.
... by: Serban
# 711 Basics to 22
1. G9=2 & J7=1 solve
2. G9=2 & G5=9 solve
3. B7=6 & D2=4 solve
4. B7=6 & H3=4 solve
5. B7=6 & J9=4 solve
6. B7=6 & C8=4 solve
