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Tough Sudoku for 18/September/2006


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

Submitted by: sudoku lover

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I'll have to take your word for it, fi, I'm afraid that I can't see the birds unless those two little spots are their eyes.
Sorry, I can't see the birds. The cat - yes; but no birds.
there is a sea lion just ready to break the surface
maen all
I don't get it. I went through this puzzle the first time and solved it no trouble. When I go the 'Solve Easy' route, I'm stumped.
Good morning to all! I can't see any birds either. Are they black birds?? Did they migrate for the winter?? Do I need new bi-focals???
15:32 after setup. I can't see any birds. Do see the rocks though, so all is not lost...
rocky mystics!
First tough I've gotten easily in awhile!
Good day to all.
5:26 after setup. White rocks usually mean birds are close!!
Possible proof of tough sudoku of 09 18 06:

1) Start at 22 filled - the given puzzle. Unique Possibilities to 27 filled. (UP 27).
2) Pair 28 at d23 forbids d479,e2f3=2 and forbids d7e2=8 UP 30
3) h6=8 == b6=8 -- b6=4 == d6=4 -- d4=4 == d4=3 -- e6=3 == e6=6 forbids h6=6 UP 31
4) More...
Step 4 in my proof is not very transparent - it is a Hidden Sue de Cox similar step:

If we chunk it with the previous step, it can be shortened to:

e5=3 == e5=25 -- {Hidden pair 25 at ef4} == b4=25 -- b4=8 == d4=3
One further note of interest:
if one eliminates step 3, one can still do step 4:
{de5=pair 36} == e5=25 -- {Hidden pair 25 at ef4} == b4=25 -- b4=8 == b6=8 -- b6=4 == d6=4 -- d4=4 == d4=3 forbids b4=149, b6=19,and forbids e4f6=3 UP 37
The rest of the steps listed will still work - just up one less until the last step.
It's taking me ages, but I am finally getting them without guessing!

I thought the picture was meat on the BBQ!
One final note of interest:
A very nice proof is also available using the almost hidden pair 89 at b46:
{Almost hidden pair 89 at b46}-- {b4=25} == {hidden pair 25 at ef4} -- f4=1 == f6=1 -- f6=7 == d6=7 -- d4=4 -- {Almost hidden pair 89 at b46}
forbids b46=9, plus perhaps some other items....
1) Start 22, Unique Possibilities to (UP) 27.
2) Naked pair 28 at d23 forbids e2=28, f3=2, d4=2, d7=28, d9=2. UP 30.
3) e2=9 == a2=9 -- a6=9 == ac6=13 -- f6=1 == f6=7 -- f78=27 == f8=9 forbids f3=9. UP 35.
4) d4=4 == d4=3 -- d9=3 == e9=3 -- e9=2 == f7=2 -- i7=2 == i7=4 forbids i4=4.
5) Locked 4s at gi5 forbid b5=4.
6) Hidden pair 48 at b46 forbids b4=125, b6=19. UP 81.
Excuse my ignorance Steve, but does 'chunk it' mean 'add it'??
In this case, Kaz - it only meant to use parts of the previous fc. 'chunk' is sort of gereric for using information we already have.
Very nice proof, Clark!
- much better then mine today. Sometimes I get hung up trying to use techniques beyond what the puzzle actually requires. Your step 3 is much, much more straightfoward then any part of my proof. Guess I should put away my elephant guns until the big game appears.
This rock looks beautiful. Is that a river at the bottom? It would be nice to go and visit. I would like to see it first hand. Nice picture. Thanks
I can only see one bird, a cormorant in the middle of the pic on the 'white rock' - like you say Dwain white rocks = birds!
I see the bird on the white rock, but is that another almost to the top on the 'pink' rock directly above that?
I know I see all kinds of things, but if you look at the center top, it looks like a face in the rock.
it can be done without resorting to ALS or AHS or even grouped
premises - just 3 simple chains:

1) Start with 22, UP 27
2) NakedPair: d32=28 => ~d974=2, ~(f3, e2)=2, UP 30
3) Locked: bc9=5 => ~h9=5
4) Locked: de9=3 => ~hi9=3
5) Locked: def6=7 => ~h6=7
6) e9=3 More...
in the above Clark gets credit - I would have skipped looking at the seemingly ineffectual
steps 7 and 8.
I had to remove a possibility by trying, until an impossibility arised. Then in a triplet consisting of 3 pairs the first(out of two possibilities)chosen lead straight to the solution!
Very well done, Pat!

Again, I apologize to all proof readers for forcing in a technique that a puzzle did not even come close to requiring - kinda like having a new toy - just want to use it somewhere!
Cyndi,glad it wasen't just me,I also saw the beak at the right as an image of a bird with a squinted eye to the left above that beak image.Now I see the bird on the white rock also.Takes a bit of study.Good picture!
Finding the birds was harder than doing the puzzle.
i dont follow clarks proof. In step 3, a3=9 is also a possibility, how is it eliminated?
The whole image looks like a bird to me - especially coming in. The cavern is the beak tucked into the bird's breast - a glistening eye at the top. The brown neck and paler body feathers.
ah!birds where??nevermind..if fi says it must be there..i think so for she is the one who took the pic..
It took a while... 46 mins to do the puzzle... but at least I spotted the bird straight away. I even see Kate's bird... sort of like a pelican I reacon!
I am new to this so just had to ask if at any point you have to guess or is there always a logical next step?
Over an hour to solve.
27:33 new to sudoku, nice way to keep mind limber
Nov. 23, 2007.
24/Nov/07 3:35 PM

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