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Tough Sudoku for 20/December/2006


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

  1 2 3 4 5 6 7 8 9
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Sudoku Solving Techniques

Submitted by: sudoku lover

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Lovely photo whoever took it.
Thanks Anne ;)
I should have realised, shouldn't I have, fi.
I concur.
'Twas the night before Christmas, when all through the Net,
There were hacker's a surfing. Nerds? Yeah, you bet.
The e-mails were stacked by the modem with care,
In hopes that St. Nicholas soon would be there.
The newbies were nestled all snug by their screens,
While visions of Java More...
That is a really cool photo!
too easy today.
Hmm... too easy today? over hour and not finish, too long time meet real tough in this page - exciting puzzle.
Great picture - and great parody!

I found today's a lot tougher than yesterday's.
5:55 Maen Nice Pic
Freaking immmmmposssssible. Whoever thought it was 'too easy' did not work the same puzzle I did! I cheated and did 'check moves' for the first time in... well, thousands of puzzles at least. And even then it took me forever.

Did not have the best of days.
fi, that's a lovely photo.
Gorgeous photo, fi. The light seems to be shining through the birds' wings.
Good afternoon to all! Beautiful picture Fi. Hope you don't mind, but I've downloaded it and may use it for a screensaver!
good mAen! Yeah this is the first time ive had to guess then run through.... I got lucky and used logic to the best of my abilities and did 15 minutes and didnt need to backtrack, but i had a far more difficult time with this one than yesterdays.. And i dont know how to do the proof method either, More...
I agree with Karen! I got 25 filled in and then spent forever, finally I had to make a guess.
I hope we get a proof on this one.
Plugged it into Simple Sudoku, which found two hidden singles and was immediately stumped. I'm no prover by any means, however these eliminations seemed to work. Maybe they'll help someone out.

On 1's: c2==c8--g5==i5 forbids g8,i8
On 4's: e8==18--h5==h7 forbids i5,i7
On 8's: b1==b3--d5==f5 forbids d1,d3,f1,f3
Got two singles, giving up :-)
Some eliminations, but no closer,
at 25 filled, 3) a2=7==e2=7--e4=7==e4=4--e8=4==i8=4--h7=4==h5=4--h5=6==h2=6, forbids a2=6
4)f4=2 or f4=4 , case f4=2:--c4=2=={a1,c4=56}--c5=6 and case f4=4:--e4=4==e4=7--b4=7==b4=6--c5=6, either case forbids c5=6
TYPO: 4's elimination should read e8==i8--h5==h7 forbids i5,i7
I ditto the other comments: this was incredibly hard today! I guessed several times.
I really do not have enough time for a proof of this monster today. Here are a few probably worthless eliminations:
1) Start at 23 filled - the given puzzle. Unique Possibilities to 25 filled. (UP 25).
2a) c1=6 == c1=5 -- c4=5 == i4=5 -- i4=6 == bc4=6 forbids c5=6
2b) {de3=6 == d1=6 -- More...
I am in absolute concurrence with everyone except Lizbaby4 today, glorious pic Fi, and a hard tough!!
Very nice poem Sarah! - (much better than the one I posted in the blog).
I've seen some references in these comments to something called 'Simple Sudoku.' What is Simple Sudoku? How can I get it? Thanks for any help anyone can give me here.
Almost 2 hours to solve
Jay: Simple Sudoku is a free program that is very helpful in solving sudoku problems. It also generates its own problems. You can download it at http://angusj.com/sudoku/
I will probably pare this monstrous proof down at a later date. Please forgive the likely typo's:

A proof for tough! sudoku of 12 20 06:
1) Start at 23 filled - the given puzzle. Unique Possibilities to 25 filled. (UP 25).
2a) b4=6 == b4=7 --{b1=7 ==d1=7 -- e2=7 == e4=7} == e6=7 More...
'Tis a bad sign when the proof does not fit in one post:

UP 30
6a) e6=3 == e6=7 -- g6=7 == g9=7 -- g9=9 == g8=9 -- c8=9 == a789=9 -- a6=9 == d9=9 forbids d9=3
6b) i9=7 == i4=7 -- e4=7 == e4=4 -- e8=4 == i8=4 -- i5=4 == i5=6 -- i3=6 == h2=6 -- h2=3 == h79=3 forbids i9=3
6c) a5=3 == More...
One step is certainly better presented as a matrix - step 6d - which is also, I think, the max depth =9 step.

Some of the other steps use a multi-chaining idea inside the forbidding chains. I have fine-tuned this idea a tad from previous proofs. Some day it will be examined in the blog.
not last. steve is the last word in sudoku.
now that's some photo!!

05/Sep/10 11:15 PM
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