#719, October 10 - October 16, 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: Dieter
#719 - initially populated cells: 22 = 27,2 %
No solver used, solver = brain, supported by self-coded EXCEL app.
basics:%
Strategy: numbers 2,4,8,9 occur most frequently, combine two cells with almost same candidates.
combinations of A2 = (1,5,7,9) and D1 = (1,4,5,6,7,8);
A2=1 & D1=1 solve.
... by: Norman Powell
AS's solver makes 8 eliminations before running out of techniques:
X-Cycles 4D1, Forcing Nets 1A7, 6D5, 6D7, 7E1, 9J7, 7E2, 7E3
My Solver solves this puzzle after the 5 Pass through Forcing Networks, which finds a solution testing 8D3 for ON.
Details:
Finned Swordfish: BD2G67 with Fin E2 for Value 4 - Eliminate 4D1 from Fin Box. [AS #1 X-Cycles]
[I have all the finned fish earlier than Forcing Networks in the list of methods, as they are fast searches on single values, faster than my implementation of X-Cycles, which is slower than my Forcing Networks, so currently appears after Forcing Networks]
Forcing Networks First Pass: The following create contradictions when ON so eliminate:
1A7 [AS #2], 6D5 [AS #3], 6D7 [AS #4], 9J7 [AS #6]
Forcing Networks Second Pass: The following create contradictions when ON so eliminate:
7E1 [AS #5], 7E2 [AS #7], 7E3 [AS #8]
Forcing Networks Third Pass: The following create contradictions when ON so eliminate:
3A7, 9B8
Forcing Networks Fourth Pass: The following create contradictions when ON so eliminate:
7A1, 7D1, 7D2, 7D3, 3F9, 3G8, 1H2, 9J4, 9J5
Hidden Single H9 = 9
Confined to Box: 7 is confined to Row F and Box 4, eliminate other 4's in Row F or Box 4.
Hidden Triplet in Colum 7 for values 4 7 9 in Cells A7, B7 and D7, eliminate other possibilities in those cells.
Confined to Box: 1 is confined to Column 7 and Box 9 eliminate other 1's in Column 7 and Box 9
Forcing Networks Fifth Pass: Makes 15 suggestions for elimination before finding a solution testing 8D3 for ON.
Solved in 2mins 11secs on VBA solver in Excel.
... by: K9Dr
Using my previously detailed self-written computer code.
246 steps. 37 Trials, 31 errors. Solving time: 0.9 seconds
... by: Frans Goosens
#719
With trial and error
Combination C3=179 and H7=139
************************************************************
C3=1 H7=1 Wrong, Undo calculation
All reset to initial position
************************************************************
C3=1 H7=3 Wrong, Undo calculation
All reset to initial position
************************************************************
C3=1 H7=9 Wrong, Undo calculation
All reset to initial position
************************************************************
C3=7 H7=1 ( G8=5 ) Solved,
Total solving time is : 89 sec.
Number of logical steps is : 12667
... by: Serban
# 719 Basics to 22
1. A2=D1=1 solve
2. A2=1 & B9=4 solve
