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Tough Sudoku for 18/September/2005


Choose a number, and place it in the grid above.

  1 2 3 4 5 6 7 8 9
Check out the latest post in the Sudoku Forum

Sudoku Solving Techniques

Submitted by: sudoku lover

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Got stuck again at 28 places still to be filled. Have to make an assumption here, and I do not like that (hate trial and error when doing logic...).
There is no way (still) to 'split' in 2 directions; and I cannot be sure that the solution I'm going to (after choosing some direction) is the ONLY one....
Okay...maybe saving the game and going in one direction...no good? Load game and go in the other dir...but still: I like logic better
There is no use timing yourself in order to report that time to others; either they won't believe or they will laugh about your slowness... Just time it for yourself... There are too many cheating possibilities and there is no one checking your puzzling...alas...
Heh! Managed it in 25. Forced to guess once. Brain turning 59 tomorrow so there's hope yet!
I didn't have to guess but it took me 29.44 minutes and I checked nearly every move - made one or two mistakes along the way but backtracked.
Had to guess onceclose to the end, didn't see any way out without guessing.
I don't believe it can be done without guessing.
Still stuck after new try...so please help.
From the start I can place two 4's, then one 2, four 7's, one 8, one 5 and a 1.
Then I fill in possibilities, find a few certainties more, but that stops at still 26 places to be determined. Have to make one assumption there and I dont want that.
Am I missing something?
4:56 {{yippee}}
GG - have a look at the 'proofs' link in the top right, it has a method of solving these tough ones without guessing.
**spoiler** in this case, when you get to the 28 places left, look at the 6's in r4c6, r4c9, r6c6, and r6c8.
Thanx pk...didn't try that link yet...but I will
Yeah, the X-Wing technique really helps. Although, I'm not totally convinced if it's a legitimate thing to do. Does it always work for you guys?

How the hell did they derive that technique logically?
GG I suppose you were here:

7 8 23 94 6 29 15 345 13
4 9 1 7 3 5 2 86 86
5 23 6 8 24 1 9 34 7
18 5 23 19 28 296 4 7 396
9 6 4 3 5 7 8 1 2
18 23 7 194 284 296 56 356 396
3 7 5 2 9 4 16 86 186
2 4 8 6 1 3 7 More...
5.32 Getting better
From here you're straight to unique solution.
(I used andrei notations, see archive 09/16 tough)
Just a try below for correct More...
Thanks, PK, for the tip re x-wing. It's a new one on me, and I'll use it!!
continued : not convincing! Tring with tab key?
Has anyone any idea how to get correct tabbing when typing a grid state on these comments?
Hi gb!
Thanks for your hints: they worked.
Now about tabbing: when you type in the text, the character set used is non-proportional (Courier in my machine). But, when it appears in the list, it is proportional, i.e. the 'point' and 'space' take just a little room. Try the underscore '_' as seperator; it takes up the same space as a figure...good luck
You get something like this:


I'm not saying its beautiful, but it looks ok, no?
I don't really get the x-wing thing. I can't see the logic behind it either. anybody with any insight would be very much appreciated...
Or rather:


I was finally able to finish a tough one without notation of any kind, and only one guess. (my hand jerked, I wanted it so badly). Its all a matter of thinking in terms of reducing possibilities as with the subtractive notation systems without actually typing in all the little numbers... I'm More...
GG : Thanks a lot for the tabbing tip.
For anyone who cares : I wrote an html proof of uniqueness for this tough. See
and feel free to make suggestions. I'd be very proud if Gath considered adding it to the 'Proofs' section of his website.
First time I finished a tough one without guessing even once!!!!!!
15:01 Was 2/3 complete, and then had to make 2 guesses. Everything slid in after that.
Hi gb, I also completed and wrote down a solution of today's tough using my notation in your improved form. I sent it to our mediator Gath, so I think he should provide you with it if you are interested. My solution is very similar to yours.
Hi andrei,
you can send me your proof at nospambrunogreco@free.fr (remove nospam before posting!). Any improvements for simplifying proof of tough 09/16 since yesterday?
13:40 cool, all 4 done for the day..now to tackle other problems that come to me...such as how to keep a teen age daughter in-line
Thanks to Leo who reread my htm file and found a typing flaw.
One suggestion for a new category of puzzles in addition to Easy, Medium, Hard and Tough: Research puzzles, where no simple elimination (%cell, %block, %row or %column, in my notation) is possible from the initial position. They may not look as symmetric as the current ones, but finding simple More...
continue: from this day's tough by performing all simple eliminations.
Agree, agree, agree with you, andrei! This first elimination step is good for lower levels, to get beginners acquainted with the basic mechanisms of the game.
Before our wizard Gath creates this new feature, one can write down (thanks to GG tip) the position after this elimination step, e-g More...
what's x-wing?
Really enjoyed this one today because I didn't have to guess once. gb thanks for the eliminations because I missed a couple first so it made me think and so I didnt need to guess. Great when that happens. Time irrelevant but 22:48 time well spent
Took a long time but no cheating!
p.s. the pictures are getting better each time I play!
Here I am brain the size of a planet and I can't solve Sudoku!
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