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## Discussion...

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## ... by: Dr. Chris

It is possible to solve this puzzle using only pencil and paper through logical analysis and a process of elimination. Columns 1, 6, and 7 contain similar numbers: 2689, 1268, and 1269. They also each have empty cells in rows C, G, and H, and two of three cells empty in rows D, E, and F, with a 2 in the remaining cell in a different row in each column. This suggests that the placement of numbers in each of columns 1, 6, and 7 will help determine the positions of the numbers in the other two columns. The only 7 among the initial clues, and one of the only two 4s, are in row C. This limits the possibilities for C1, C6, and C7 and suggests that this row will unlock those columns. C1 must be 1, 3, or 5; C6 must be 3, 5, or 9; and C7 must be 3, 5 or 8. Since numbers cannot be duplicated in the row, there are only 13 possible combinations. Each combination provides enough material to work the puzzle forward to a falsity or a solution. I sensed intuitively that C7 could be 8, so I began with combinations in which C7=8, in ascending order: 1,3,8; 1,5,8; 1,9,8; 3,5,8 … and that fourth combination led to the solution.

## ... by: Serban

correction
Row C n=2 e=7 Nc=64 Ns=25 =>C3=1 C6=5 C9=2 basics to 39
X-chain x=1 =>H1<>1 x=4 =>[ GH1,DE3 ] <>4 x=5 =>[ F1,FJ4]<>5
=>F4=J3=4 D3=7 solve

Number 8 n=3 e=6 N=19 Nc=21 Ns=5 B3=C7=D8=E5 =G4=H8 =8 Basics to 37
1. X-chain x=7 =>[ J1,EF4,F5 ,GH6 ]<>7
2. XY-wing 479 [ D39,E8] =>E13 <.>7
3. W-wing 35 [J2,E3] =>D2<>3 =>D2=5 solve

## ... by: Serban

Row C n=2 e=7 Nc=64 Ns=25 =>C3=1 C6=5 C9=2 basics to 39
X-chain x=1 =>H1<>1 x=4 =>[ GH1,DE3 ] <>4 x=5 =>[ F1,FJ4]<>5
=>F4=J3=4 D6=7 solve

e

625 178 943
948 326 157
371 945 862
257 619 384
463 587 291
189 432 576
792 863 415
516 294 738
834 751 629
works

4E8

## ... by: Frans Goosens

With trial and error

I have decided to calculate cell A3 and cell G9 all combinations
A3=123 G9=139

A3=1 G9=1 Wrong. Undo calculation
A3=1 G9=3 Wrong, Undo calculation
A3=1 G9=9 Wrong, Undo calculation

A3=2 G9=1 Solved,

#299
982 764 513
731 852 496
645 139 728

593 476 182
824 591 367
176 283 945

469 327 851
257 918 634
318 645 279

## ... by: SteveJ

Requires two guesses. No single guess is sufficient, or a single level of recursion would have solved it.

stevej@bsd84:~/sudoku % ./sudoku -f ./puzzles/un-1.txt -s 2 -r -D 1
*** ./puzzles/un-1.txt not solved, sane! Solution failed!
Iterations: 2. Changes: Rows 136, cols 99, squares 32, tot 267
dfs: cells 59 vals 264 rec 264 bt 264 ns 0 mrl 1
stevej@bsd84:~/sudoku %
stevej@bsd84:~/sudoku % ./sudoku -f ./puzzles/un-1.txt -s 2 -r -D 2
>>>>>> ./puzzles/un-1.txt solved, sane!
Iterations: 13. Changes: Rows 205, cols 180, squares 83, tot 468
dfs: cells 3417 vals 14418 rec 13819 bt 13818 ns 598 mrl 2
stevej@bsd84:~/sudoku %

## ... by: Frans Goosens

U.C means Undo calculation.
I have decided to calculate the cells C2 and G8

C2=4 G8=1 Wrong. U.C
C2=4 G8=2 Wrong, U.C
C2=4 G8=8 Wrong, U.C

C2=5 G8=1 Wrong, U.C
C2=5 G8=2 Solved,

#294
623 471 859
478 659 231
159 283 647

245 916 378
731 528 964
896 347 512

384 195 726
517 862 493
962 734 185

## ... by: Don

Simplest Unsolvable Sudoku

1 0 0 0 0 0 0 0 0
0 0 0 1 0 0 0 0 0
0 0 0 0 0 0 1 0 0
0 1 0 0 0 0 0 0 0
0 0 0 0 1 0 0 0 0
0 0 0 0 0 0 0 1 0
0 0 1 0 0 0 0 0 0
0 0 0 0 0 1 0 0 0
0 0 0 0 0 0 0 0 2

Many variations possible.

Don

## ... by: RICHARD ALLAN

Even using logic,I had to make about ten 1/2 guesses and the puzzle still took me about 3 hours to solve,what a stinker.The 6x6 puzzle yesterday was easy compared to this one.Once you found all the 5s,the problem was solved.

## ... by: Duder Sud

Use 7 in A6 then use solver

mjanbju

## ... by: Sarah

Yes it's funny how dilemmas similar to this one start looking unbelievably insignificant when compared to the world events. Another part of the cold-war, the actual authentic war that erupts, Russia-China fuel deal axis... Nonetheless here we are with your socialmedia dilemmas, - can we see the world has transformed? I'm not stating what you write about is unnecessary, I'm indicating that the certain degree of detachment is healthful. Thanks, Sarah @ http://phyto-renew350e.com/
Sarah

## ... by: jjflip

dude, your slover blows! its WAYYYYY to complicted. i tried another solver and it worked pretty quick

## ... by: Solved? Or Not..

Is this right?
[6, 2, 5, 1, 7, 8, 9, 4, 3]
[9, 4, 8, 3, 2, 6, 1, 5, 7]
[3, 7, 1, 9, 4, 5, 8, 6, 2]
[2, 5, 7, 6, 1, 9, 3, 8, 4]
[4, 6, 3, 5, 8, 7, 2, 9, 1]
[1, 8, 9, 4, 3, 2, 5, 7, 6]
[7, 9, 2, 8, 6, 3, 4, 1, 5]
[5, 1, 6, 2, 9, 4, 7, 3, 8]
[8, 3, 4, 7, 5, 1, 6, 2, 9]

Thanks!

## ... by: JULIEN

ok I did this Sudoku but there is a mistake and it is why it is impossible, the 9 in b1 should be at c2 therefore 7 c2 should be at b1. here the solution without the misplacement of this 2 number:
652-138-947
734-296-158
198-745-326
247-619-583
561-483-279
389-572-461
923-864-715
416-957-832
875-321-694

this is the one and only solution, the person who did invent this Sudoku put an error to be impossible...
but now with this 2 number put back at their right place now it is possible ( see result above).

it took me 4 hours but I find it.

roncq@hotmail.co.uk

## ... by: newby

first finding all the 3 number sets, (3,5,8)C7 was most interesting as it offered the most paired sets using 8C7.
The using pairs (2,9)J8,J9; (1,8)H9,G8; (4,7)G7, H7
9J9, 8H9, 7H7 solves!!

## ... by: Enes

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## ... by: marcel hendrix

This one takes 0.061 ms to solve. That is not much, compared to 2.5 ms for Arto Inkala's idea, or some samples from sudoku17.txt (326 ms for line 527, 989 ms for line 6361).

-marcel

## ... by: samuel

6 2 5 1 7 8 9 4 3
9 4 8 3 2 6 1 5 7
3 7 1 9 4 5 8 6 2
2 5 7 6 1 9 3 8 4
4 6 3 5 8 7 2 9 1
1 8 9 4 3 2 5 7 6
7 9 2 8 6 3 4 1 5
5 1 6 2 9 4 7 3 8
8 3 4 7 5 1 6 2 9
is that right?

## ... by: Strmckr

a question for andrew if he reads this:

als-xy rule:
instead you are using mike barkers
Deathblosom which is also an als-xy rule but limited in its effectiveness through restrictions in its initial design.

## ... by: Jean-Claude

Very nice picture, strmckr, I have to buy colour pens to get there !

In order to let better understand your method, could you draw the equivalent picture for unsolvable # 27.

Many thanks,

Jean-Claude Casari
casarijc@hotmail.fr

## ... by: strmckr

ronk: posted a great pic of the logic set.

http://i41.tinypic.com/16kqmvd.jpg

after the BI BI pattern it solve using singles, 3 locked pairs 3 locked candidates and 1 skyscraper

I'm thinking the remote pairs is a bit excessive :P

## ... by: JC Van Hay

Typo in previous post (C1 -> C3). Thus :

2 very simple steps to solve the puzzle :

#1. Exocet pattern : see ttt's post in http://www.sudoku.org.uk/SudokuThread.asp?fid=4&sid=11320&p1=1&p2=17.
#2. Remote pairs : RP(35)C3B5 => +7r1c5; stte.

## ... by: JC Van Hay

2 very simple steps to solve the puzzle :

#1. Exocet pattern : see ttt's post in http://www.sudoku.org.uk/SudokuThread.asp?fid=4&sid=11320&p1=1&p2=17.
#2. Remote pairs : RP(35)C1B5 => +7r1c5; stte.

## ... by: Jean-Claude

I have no logical solution: there no bi-value cell.

Help !

## ... by: Nick the Geek

Wow! Immediate "Rooks!" (Run Out Of Known Strategies)! I'll give it a go today.

## 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

I had also included David Filmer's puzzles before #200 in the list.