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Tough Sudoku for 31/January/2007

                 
                 
                 
                 
                 
                 
                 
                 
                 

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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I knew it would be one of your photos, Keyan.
What an absolutely stunning photo.
great pic timing had to be just right.
7:17 Maen
Finally, managed to reach the photo alone. Doubtless, someone say this was especially easy, but it'll not diminish my sense of achievement.
8:42 Maen Interesting picture... what city is that?
22:34 I found it fairly easy & straightforward. It took me 5 minutes to get the puzzle set up. How is it possible to solve in 7 or 8 minutes?
Hi everyone and nice mAen to all.
Good shot Kayan but where the sun and moon .We should see one at least.
Nice picture and easy hard.
Sorry, I ment easy tough ,thanks
I'm with you Joe...this one was difficult. Great photo.
8:28 today. Nick - I do not count the time it takes me to set it up.
I'm curious, where was this photo taken? Its a great shot.
Have a great day/night, everyone.
No guesses today.
When I see a full moon like that at sunset it always reminds me of the story 'East of the Sun and West of the Moon'
maeN all!

It's San Francisco Bay, looking toward Oakland. The moon was rising on a clear evening with a particularly rich red-gold sunset.

Easier tough than usual?
17:52; sad to admit that I used 2 guesses!
Oh, and this gorgeous shot HAD to be SF!
This puzzle unlocks easily with 2 Y wing styles:

A proof for tough sudoku of 01 31 2007:

1) Start at 23 filled - the given puzzle. Unique Possibilities to 28 filled. (UP 28).
2) Pair 17 at c56 forbids c2ab56=1 and forbids c28ab56=7 UP 30
3a) Hidden par 38 at df1 forbids More...
In an attempt to remain on topic with the blog, setp 4b) can also be done at depth 4 using an Almost Locked Set in a chain:

h3=1 == h3=7 -- g2=7 == {pair 24 at fg2} -- e2=24 == e2=1 forbids d3=1

In general, there will be such a relationship - more than one fc to forbid equivalent More...
That's a very clever photo of an attractive subject. Well done Keyan.
I was just at a party in December with almost the same view from the windows of the house, although not the same lovely moonrise. Instead, the lights on the Bay Bridge made for a pretty picture.
Great photography Keyan.
[Why do I think I hear Tevya singing?] How can one not count setup in one's time? Isn't that 9/10 of it? Could we have parameters for times, or times with codes so others know what they mean?
Apparently easy tough; I solved it. Though I think I'm on my way to understanding chain theory. Steve, some of your steps have improved my ability to solve logrithmically. Thanks!
Beautiful picture Keyan! Steve, your blogs are helping this blond really solve and not just guess!
Good afternoon to all! Another great photo Keyan. Well done!
Keyan, that photo is what I would class as 'painterly'. A truly beautiful shot, and I wondered what camera settings you would use for it.
Why do you say 'going back'? Is this your home town?
A lovely photo. Are you also a painter, Keyan? You have a great eye for composition.
Had to guess once. Not bad for a tough, though. Waiting for a snow squall here in COLD PA. Brrr!
Maen all! A momentous day today! My Leia started big school! My baby is all grown up!
Ok... tougher to caption than to solve:

UFO drag racing along the river. Servilix bet Regulonij 7.5 gigagallons of rare Yerlix ionized Jryogen (pastuerized - NO BGH! Use for either UFO fuel, galactic cocktail mixer, or sell to Earthlings as drywall compound) that he/she/it* couldn't fly More...
Hi, Kate,

I was an exchange student there in 72-73.
Was housed in Killara, Castle Crag, Kensington, and Rose Bay. Attended Killara and Sydney Girls.
Sponsered by Dulwich Hill Rotary Club.
Got taken to Apia Club by one of the sons, but now that I think of it more I think it was More...
I am starting to worry about Wagdy's eyesight, not grammar. On the easy page, Wagdy thought the sky was green. Dude, tell me that was just poor word choice! I have said 'verde' when I meant 'azul' when speaking Spanish so I can relate...But the moon is clearly visible in this photo.
'Oh, I wanna be there, in my city by the bay....' ~ Journey
I love doing these puzzles....I wish I didn't have to guess at the tough ones though lol Am I the only one who does not understand Steve's proof? I'm too new to Sudoku's I guess.
Another stunning photo Keyan.
i totally do not get steve's proof...they make my head hurt! I'm just not that logical!
Hi Jeanine! Lane Cove is across the harbour on the way to Roseville but I grew up in Croydon and went to high school in Ashfield. I had (and still have) several Italian schoolpals and the Apia club is part of my highschool vernacular - early 80s vintage. There's a large cemetery/crematorium in More...
Kate, you have me worried. You just talked about the crematorium/cemetary and how wonderful it was to be an exchange student. Was that Harry Potter's school on the previous sudoku?
Kate, that really brings back memories! and who cried most, you or her?? My 'baby' went off with a handwave, but I had to sit for 10 mins before I could drive home!!
16:08 To SpadeScorpio and sherri just give it time and go back and look at the older Blogs. 'Simple Sudoku' helps too

Not too bad. Primrose path of easy reductions led to an empty rectangle, steve's y-wings and then some coloring got it cracked. I wonder if coloring bivalue chains is the same thing steve's always talking about without the boolean formulas? it does eliminate (forbid?) candidates. Is all that More...
Hi Morgan!
The theory is presented to provide a logical basis for the technique. Furthermore, it reveals the equivalence across techniques. Of course, it then serves as a language tool.

Coloring bivalue chains is a bit less general than the way I use forbidding chains. Nevertheless, the ideas can be presented in mathematically equivalent fashion.
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