#643, April 26 - May 2, 2025:
The Weekly 'Unsolvable' Sudoku Puzzle
by Richard Kröger, guest compiler
|
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).
|
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: BGH
Correction
As everyone in this group already knows, "forcing chains" rarely go anywhere in an unsolvable.
The strategy I use is: the general approach for a pair is one of the digits will show a continuation pathway. The other digit in the pair will lead to a contradiction. This is a general observation for either "cell forcing chains" or "unit forcing chains" in an unsolvable. Discard the digit that leads to a contradiction then explore the continuation.
... by: BGH
If everyone would present their solution in steps, it would be easier to validate their solution.
I for one would very much like to learn from your efforts.
For example, I added % complete per Deiter suggestion.
Also, when there's a choice, the rationale for the choice would be interesting.
If you have a new technique, give us a bit more details, i.e.
Block Digit Vectors (BDV): where a choice in one block solves for a digit in several other blocks.
This technique is particularly useful as a starting strategy where the puzzle is very short on any of the usual opportunities. I almost always start an unsolvable w/ BDV. Or unit pair forcing chains.
... by: Serban
# 643 Basics to 21
1. Number 1 n=3 e=6 N=24 Nc=21 Ns=21 => D9=1 basics to 42
1a. if G2=3 solve
1b. if C2=4 solve
1c. if B1=5 solve
1d. if C3=8 solve
1e.if B3=9 solve
... by: BGH
#743 Correction
Step 3
4 at F5...
... by: BGH
#743 4 Step Solution
1. 7 at I3 solves for 7 in 4 Blocks PLUS lots of cleanup along the way. (Block Density Vectors) then
2. 8 at I4 is part of a pair (68) in quadruple in block R3C2 (The 6 leads to contradictions) then
3. 8 at F5 is part of a pair (48) in a triple (478) in Row F (The 4 leads to contradictions) then
4. 8 at C3 is part of a pair (48) in a triple (348) in Row C (The 4 leads to contradictions) then
Convergence.
... by: Frans Goosens
#643
With trial and error
Combination C2=349 and I8=259
************************************************************
C2=3 I8=2 Wrong, Undo calculation
All reset to initial position
************************************************************
C2=3 I8=5 No solution, Fixed
Combination 2-digits cells
B4=6 Wrong, Undo calculation
B4=9 Wrong, Undo calculation
All reset to initial position
************************************************************
C2=3 I8=9 Wrong, Undo calculation
All reset to initial position
************************************************************
C2=4 I8=2 Wrong, Undo calculation
All reset to initial position
************************************************************
C2=4 I8=5 ( A6=9 ) Solved,
Total solving time is : 98 sec.
Number of logical steps is : 12092
... by: Serban
# 643 Basics to 21
1. Number 1 n=3 e=6 N=24 Nc=21 Ns= => D9=1 basics to 42
1a. XY-Wing-349 [E47,D8] => D46 <>9 => E4=9 OR D6=8 basics to 66
1b. W-wing 58 [G3,B5] => B3 <>5 => G3=5 solve
2. C(3,4,9) D9(1,2,5,7,8,9) N=9 Nc=18 Ns=7 => C2=4 & D9=1 solve
... by: Dieter
#643 - basics: %
populated cells: 21 = 26%
strategy: try to get hidden pairs or triplets. No solver used.
combinations of A4 = (3,6,7,9) and A9 = (2,6,7,9)
5 matrices left after basic checks.
start with A4/A9 =(7/6), 58% populated: solves with A5=5
... by: Serban
# 643 Basics to 21
1. Row B n=3 e=6 Nc=24 Ns=18 => B3=9 & B5=8 solve
2. Box 4 ABC456 n=2 e=7 Nc=38 Ns=13 => A6=9 & B5=8 solve
