#703, June 20 - June 26, 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: Norman Powell
Solvable with Forcing Networks, finishing with Solving Networks finding the solution equivalent to Bowman's Bingo in this solver. Here are the Solver's notes, NB the Columns are lettered and the Rows number, and should be read from the bottom up, Alternative Inference Chains is allowed to run before Forcing Networks, since Forcing Networks would eclipse all Alternative Inference Chains eliminations:
Forcing Networks Solution Found - Eliminate all Trial OFFs
Forcing Networks Value 3 in Row 2 Column H cannot be ON so eliminate
Contradiction: More than one Value 1 ON in Row 4
Forcing Networks Value 5 in Row 5 Column C cannot be ON so eliminate
Contradiction: No possibilities left for Value 5 in Row 2
Hidden Singles In Row 6 H6 = 4.
Forcing Networks Value 4 in Row 6 Column E cannot be ON so eliminate
Contradiction: More than one Value 7 ON in Box 4
Forcing Networks Value 1 in Row 6 Column H cannot be ON so eliminate
Contradiction: No possibilities left for Row 2 Column C
Forcing Networks Value 8 in Row 6 Column F cannot be ON so eliminate
Contradiction: More than one Value 7 ON in Row 4
Forcing Networks Value 8 in Row 6 Column D cannot be ON so eliminate
Contradiction: More than one Value 1 ON in Row 4
Alt. Inf. Chains Value 4 of Cell D6 has weak links to both ends of the chain
This possibility would cause a contradiction - Eliminate
X-Cycles Value 9 of Cell E6 has weak links to both ends of the chain
This possibility would cause a contradiction - Eliminate
Confined to Box Value 6 is confined to Column D and Box 5.
Hidden Singles In Row 3 D3 = 1.
... by: Dieter
#703 - initially populated cells: 22 = 27,2 %
No solver used, solver = brain.
basics: C4=1
populated cells: 23 = 28,4 %
Strategy: numbers 1,4,6,9 occur most frequently, combine two cells with almost same candidates.
combinations of G1 = (1,2,4,6) and H7 = (1,8,9)
G1=2 & H7=1 solve.
... by: Robert
This one can be solved using only very basic techniques (locked sets/hidden locked sets, box/line reduction), and forcing nets. The candidates to be eliminated (after application, often repeatedly, of the basic techniques) are:
Row 1 Col 1 Val 5
Row 1 Col 6 Val 8
Row 2 Col 1 Val 5
Row 2 Col 5 Val 2
Row 2 Col 5 Val 7
Row 3 Col 8 Val 4
Row 1 Col 9 Val 2
Row 1 Col 7 Val 5
Row 1 Col 9 Val 7
Row 2 Col 3 Val 9
Row 2 Col 8 Val 3
Row 1 Col 6 Val 2
Row 1 Col 6 Val 7
Row 1 Col 2 Val 5
Row 1 Col 1 Val 2
Row 1 Col 1 Val 7
... by: Serban
# 703 C4=1 Basics to 23
1. C8=6 & [D8=1 OR B8=2 OR H8=7] solve
2. D8=F1=1 solve
3. B7=F9=3 solve
4. E7=H1=5 solve
5. A7=J2=8 solve
