#690, March 21 - March 27, 2026:
The Weekly 'Unsolvable' Sudoku Puzzle
by Andrew Stuart
|
WARNING: This is the Weekly 'Unsolvable' Sudoku, rated above 'Extreme'.
Due to improvements in the solver since this puzzle was published this puzzle
can now be solved by the solver. Which is a good thing.
|
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
#690 - initially populated cells: 26 = 32,1 %
No solver used, solver = brain
basics: C6=8
populated cells: 27 = 33,3 %
Strategy: find two cells with almost the same candidates
combinations of D4 = (1,2,4,8,9) and E4 = (1,2,3,4,7,9)
D4=1 solves
... by: osiguy
I think Andrew is going to announce that Sudoku is basically solved here soon. Only his and my solver can solve it 100% Andrew the world is waiting for you to announce it! Or at least how close you are to confirming it! Both our solvers can literally do it! :-)
Based on emails with Andrew, he just needs to decide on how to announce it or when he feels comfortable announcing it but the data is there! It's solved!
... by: Serban
690 C6=8 Basics 27
1. H9=3 solve
2. H8=5 solve
3. H2=6 solve
4. D1=8 OR F4=8 solve
5. A3=2 & C9=4 solve
... by: Dieter
#690 - initially populated cells: 26 = 32,1 %
No solver used, solver = brain.
basics: C6=8
populated cells: 27 = 33,3 %
Strategy: numbers 8,6 occur most frequently, combine two cells.
combinations of D1 = (2,4,8,9) and G1 = (2,6,8,9)
D1=8 & G1=9 solve.
... by: osiguy
I think if you exclude forcing chains you can open this up by just doing two bowmans bingo at first:
larsdoku "000040080008630900700500600076000300500000008001000460003005001007089200040010000" --exclude fc,fpc,fpce,fn --steps
Solving: 00004008000863090070...
──────────────────────────────────────────────────
# 1 R3C6=8 [crossHatch]
# 2 R5C7=1 [BowmanBingo]
# 3 R1C7=5 [BowmanBingo]
# Rest is L1/L2
... by: osiguy
Ok from the stall point with my analyzer larsdoku bd=S9B8n8n849h042d2r082g0s110806032d092t2k070n7w057o0h060v0wbg070643840t038584057s7w9lac1s2b9g087sb801d09w2g04069wc4c4032k38055u9m011f1v070u0809021a26b804842g013c6a9iae
I see a series of eliminations that leads to a placement that opens the board, i hope this is helpful! There is another series of eliminations before a second forcing chain.
════════════════════════════════════════════════════════════
DETAILED SOLVE LOG (10 rounds)
════════════════════════════════════════════════════════════
Round 1
XWing 4 eliminations
R1C2: 6 | R1C9: 6 | R9C8: 6 | R9C9: 6
X-Wing: 4 eliminations
Round 2
ALS_XZ 1 elimination
R9C1: 7
ALS-XZ: 1 eliminations
Round 3
KrakenFish 2 eliminations
R5C6: 2 | R5C8: 2
Kraken Fish: 2 eliminations
Round 4
ForcingChain R5C7 = 1✓
Notes before: 1 7 → placed 1
Forcing Chain: all branches from bivalue cell lead to 1@R5C7
