#721, October 24 - October 30, 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.
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Supporter Members have access to all these puzzles going back to number 1.
Discussion...
Post an idea here...
... by: Norman Powell
AS Solver:
1) Hidden Pair: Values 1 & 3 in Box 7, Cells J1, H2, eliminate other values from those cells.
Forcing Nets - Eliminate 2) 1B2, 3) 1B5, 4) 9B5.
Runs out of strategies.
My Solver, starts as AS then finds a solution using Forcing Networks during third pass:
1) Hidden Pair: Values 1 & 3 in Box 7, Cells J1, H2, eliminate other values from those cells. [AS#1]
Forcing Networks First Pass: Eliminate 1B2 [AS#2], 1B5 [AS#5], 5B5, 9B5[AS#4], 6D1, 4D3, 4G1, 6G7, 6H3
Solved Cell G1=6
Forcing Networks Second Pass: Eliminate 8D3, 7D9, 8 E3, 7F1, 8G2,
8H3 cannot be OFF so eliminate other possibilities in H3, values 6 & 9
Possibilities forced to the same state when 8C8 is ON or OFF so eliminate 8G4 which is forced OFF in both states
Eliminate 9H3, 8H4, 8H5, 9J5
Solved Cell H3=8
Forcing Networks Third Pass: Suggests 4 further eliminations before finding the solution while testing 8F9 for ON.
Excel VBA solver modelled on AS's.
... by: Dieter
#721 - initially populated cells: 22 = 27,2 %
No solver used, solver = brain, supported by self-coded EXCEL app
basics: H2/J1 = 1/3 or 3/1
Strategy: numbers 2,3,6,7 occur most frequently, combine two cells with almost same candidates.
combinations of H8 = (2,3,9) and H9 = (3,6,7,9);
H8=9 & H9=7 solve.
... by: Dieter
#721 - initially populated cells: 22 = 27,2 %
No solver used, solver = brain, supported by self-coded EXCEL app
basics: H2/J1 = 1/3 or 3/1
Strategy: numbers 2,3,6,7 occur most frequently, combine two cells with almost same candidates.
combinations of D1 = (4,6,7) and F1 = (2,3,4,7);
after basic checks 3 matrices left: 7/2, 7/3 and 7/4.
D1=7 & F1=3 solve with A6=2.
Optimization:
A6=2 & F1=3 solve.
... by: Serban
# 721 Basics to 22
1. J5=5 & J6=9 solve
2. A4=5 & A6=2 solve
3. B8=1 & A6=2 solve
4. J9=3 & A6=2 solve
rg