This is the hardest one I have ever worked on. [Update 10/24/2006: thanks to you all reading this post and posting other links, I have since found more interesting ones -- look through the comments - the one posted on October 9th appears to be pretty hard, though maybe I just made a mistake on it]   When we were with my parents last weekend, we found out that my parents are big Suduko fans, and do quite a number of them during the week, in different papers, page-a-day calendars, etc.

Dad was trying to find a hard one for me, and found a "six star" one, that turned out to be not that hard - reasonably difficult, but similar to what I had seen before.  We had been trading secrets of how we figure out puzzles the fastest, and he liked one of my starting first-pass rules, but then found a puzzle that using his rule, and then my rule still did not find a single number during the first pass.  After that, the only step I know is to start writing down all possibilities, and it is sort of interesting to see how fast you can narrow them down, it is a little too brute force for my taste, and so I am not interested in it as much.

Once you get one number on this puzzle, it is a normal, reasonably difficult sudoku, the trick is getting the first number.  So, I would be interested in hearing how long it takes you to get the first number filled in, and if it doesn't take you that long, what is your strategy, because it must be different than mine.

(see my comment on 11/27/2006 for the original puzzle)

Edit: Don't read the comments if you are interested in solving the puzzle, wait until after you are done, or at least until after you have found the first square -- which Linda and I probably had the same first square, so that leads me to believe that everyone might have to start at the same point.

Posted by Jon Daley on April 28, 2006, 9:21 pm | Read 91520 times
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Solved the whole puzzle in just over 8 minutes - using the computer program Mark wrote. No other puzzle has ever been close to a minute. That convinces me not to try it on my own.
Posted by Nancy on April 28, 2006, 10:25 pm

I was the last in our family to become addicted to Sudoku puzzles, but when I did, I discovered something interesing: 1-stars weren't worth doing, 2 were somewhat interesting because I could do them "in my head," but 3, 4, 5, and 6 stars were pretty much equally difficult! That's not to say I find them easy, just that a 3 is at least as likely to be difficult as a 6. The distiction between "solvable" and "impossible" has been whether or not I make a mistake, since if I do I'm not usually inspired to start over again. I haven't done all that many, however, because I stopped when we stopped getting the daily paper. So this was a nice challenge.

I can't say how long it took me, because I was doing other things (like reading Janet's updates) at the same time. But I didn't find it particularly challenging. The first blank I filled in was the rightmost column, 5th row. After that the middle square of 9 came fairly quickly, and it was all slogging from there.

You won't like my method, however, because it starts with writing all the possibilities in each square. (The order in which I fill in the possibilities depends on the particular set up, so it's not all just slogging.) Once I've done that, certain things jump out at me, such as unique pairs. In this case, I could see that the first two rows of the rightmost columm could only be 5 or 9, so one had to be 5 and the other 9, thus when I found that the 5th row of that column could only be 3, 5, or 9, I knew it had to be 3.
Posted by SursumCorda on April 29, 2006, 6:37 am

Um, Jon -- you might want to censor my answer, since I just realized it's somewhat of a spoiler to someone who wants to do the puzzle....
Posted by SursumCorda on April 29, 2006, 6:43 am

Yes, the 5 and 9 on the right column was my first thing as well. But, I had to fill in the top three boxes with all possibilities before I discovered that. My first filled in square was either the same square that you did or else the very bottom row in that column.

I wonder if you find the 3 through 6 stars to be equally difficult because you don't do any "first pass" rules to quickly write down stuff. The level of stars is different depending on which book/paper the puzzle is in, so I don't really pay attention, other than not doing the "easy" puzzles like you said.
Posted by jondaley on April 29, 2006, 8:50 am

I sort of do a first pass, because I don't fill my grid of possibilities in numerical order, but start with whatever numbers I think will give me the most information, eliminating some of the possibilities. But I do move quickly to writing in the possibilities, since on anything over 2 stars I've found I have to do that eventually anyway. I need an eraser as much as a pencil this way, but marks on paper actually stay there unless erased, unlike my mental images....
Posted by SursumCorda on April 29, 2006, 10:43 am

Oh - do you go through all squares filling in wherever a "1" can go? I do it the other way, going through each square filling in all possibilities for that square. Your way (if it is your way) might be quicker, since it might be easier to skip more squares without a whole lot of thinking.

I have always done an explicit first pass without writing down anything except for numbers that I am sure of. It is a relatively quick process. Last weekend, I developed a new strategy, where I write down little numbers if I know that a "5" has to either be in this square or that square, within any group of 9, though more likely in a box than in a row or column. That rule is how I got the 5 and 9 in this particular puzzle.

I don't like pencils all that much for any kind of writing, so when doing a suduko, one sub-goal is to write down as little as possible, because I won't be able to erase it.
Posted by jondaley on April 29, 2006, 11:19 am

The first thing I do is copy the puzzle to my own grid, which I print out; it's a regular sudoku grid but bigger (most of an 8.5x11 page) to give me more room for writing. (There is not enough room in those tiny boxes in the version in the Orlando Sentinel.) I subdivide (mentally) each blank square into 9 parts, 3 rows of 3: 123/346/789. I generally start with the number that appears most often in the puzzle, and fill in where the missing occurences of that number might be. For example, if there are four 3's given, I write a "3" (in pencil) in the upper right hand corner of every small square where a 3 might go. Then I do the same thing for the other numbers, marking 5's in the middle, 7's in the lower left, etc. (I could just use dots, because I indicate each number in a unique place, but writing the number itself helps reduce errors.) I'm making this sound complicated, but it is a pretty quick process. There's still a lot of work to do, usually, but at least I can see better what I'm doing.

I know Porter works in an entirely different way, and so do you; I wonder how many different approaches there are? Then there's always Mark's.... :) I wonder if any are insoluble?
Posted by SursumCorda on April 29, 2006, 12:28 pm

Insoluble: The puzzles or the methods?
Posted by jondaley on April 29, 2006, 12:52 pm

Puzzles. Eight minutes is a long time for a computer, but the puzzle was solved. I don't suppose they intentionally publish puzzles for which there is no solution, but it seems that a mis-print might make it impossible.
Posted by SursumCorda on April 29, 2006, 1:06 pm

Right, it would have to be a misprint. I did one at Thanksgiving where I think there were two answers. Janet also said something about sometimes having to guess, and just see if it works, and I think that can only be needed if there is more than one solution (or incomplete checking on the person doing the puzzle).

At Thanksgiving, I didn't feel like re-doing the whole puzzle to see if it would have worked the second way too.
Posted by jondaley on April 29, 2006, 1:20 pm

Don't believe a word I say. Or at least take it with a grain of salt. I was about to post and say "Am I not a member of the family anymore? since I'm not into sudoku puzzles, but your last post reminds me that I did do a few while on vacation with family. I'm not sure I even remember how to do them now, though. Wow, how short my memory is!
Posted by Harp on May 1, 2006, 6:52 am

Ah, but you're still correct. I said I was the last in our family to become addicted to sudoku, and doing a few puzzles doesn't count as an addiction. Do you think the name is Japanese? I could spell it in katakana!
Posted by SursumCorda on May 1, 2006, 6:59 am

I went looking for the story on Suduko creation - some American guy, but I can't find the story at the moment, but I found this puzzle solver, so I typed in the puzzle - it took less than a second to solve it.

It does have an interesting feature of checking whether there is more than one solution to a puzzle, so presumably that means that happens sometimes.
Posted by Jon Daley on May 1, 2006, 7:42 am

According to Wikipedia, the first Sudoku puzzle was published in America but it first caught on in Japan. Sudoku is a shortened version of the title in Japanese, so that much is for sure Japanese. I didn't see an inventor's name.
Posted by Harp on May 2, 2006, 2:46 am

That's interesting. In one of my parents' Sudoku books, it had a story about some American guy thought it up, and just thought it would be fun to name it a Japanese name or something.
Posted by Jon Daley on May 2, 2006, 6:18 am
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