#720, October 17 - October 23, 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
#720 - initially populated cells: 23 = 28,4 %
No solver used, solver = brain, supported by self-coded EXCEL app
basics: G6=9; F1<>8
populated cells: 24 = 29,6 %
Strategy: numbers 9,2,3,5,8 occur most frequently, combine two cells with almost same candidates.
combinations of B5 = (1,3,8) and B6 = (1,5,8);
B5=1 & B6=5 solve.
... by: Frans Goosens
#720
Met -trial and error-
Combinatie A3=147 en G7=178
************************************************************
A3=1 G7=1 Geen oplossing, Gefixeerd
Combinatie 2-cijferige cellen
A1=4 Fout, Ongedaan gemaakt.
A1=9 Fout, Ongedaan gemaakt.
Alles teruggezet naar begin stand
************************************************************
A3=1 G7=7 ( A1=7 ) Opgelost,
Totale oplostijd is: 77 sec.
Aantal logische stappen is : 8681
... by: Serban
# 720 G6=9 Basics to 24
1. B8=8 & F1=9 solve
2. B8=8 & F3=3 solve
3. B8=8 & E3=5 solve
... by: Norman Powell
AS's Solver:
1) Singles G6=9
2) Pointing Pair: eliminate 8F1, 8F3
3) X-Cycles: 3D1
Forcing Networks 4) 6F6, 5) 6E2, 6) 4E2, 7) 4E3
Runs out of strategies.
My Solver: Solution found in fourth pass of Forcing Networks.
Hidden Single G6=9 [AS#1]
Value 8 is confined to Row E and Box 4, eliminate 8F1 and 8F3 [AS#2]
Finned Swordfish: Value 3, CDH347 with Fin F3 - Eliminate 3D1 [AS#3]
[Finned Swordfish is earlier in my list of strategies, as it is simpler and faster than X-Cycles]
Forcing Networks First Pass: 2D1, 4E2 [AS#6], 4E3 [AS#7], 6F6 [AS#4], 7J9,.
Forcing Networks Second Pass: 6E2 [AS#5], 4G1, 4J6
Forcing Network Third Pass: 4A3, 6D1, 2D2, 6D2, 1D6, 6D6, 6E1, 6E3, 2E4, 7G4, 6H4, 6H6
Solved Cell D6=2
Hidden Singles J6=6, G4=5
Forcing Networks Fourth Pass finds solution while testing 7A1 for ON.
Solver VBA for Excel 1 min 4 seconds
