January 30, 2007

Stupid Sudoku…

I’m sitting at the back of metric spaces and I’m bored. So what do I do? Why the Sudoku from this weeks Boar!

After a few minutes I realise something seems very familiar about this Sudoku…I still have my old copy of last weeks Boar in my bag, so I check it and sure enough it is the same Sudoku! Which I couldn’t solve last week…bummer.

Still, I try it again, I get the idea that a clean start will help me out, so I resist the temptation to just jump to where I was before. And sure enough after a while I look over and notice I have got a bit further. But I soon get stuck again and despite a lot of checking I can’t see any other moves.

Now I consider myself pretty decent at Sudoku, so I wasn’t going to let this defeat me (read: stubborn). But it just seemed there wasn’t enough information there. So I got out my old copy from last week and just tried guessing a few numbers. Much to my shock, I worked it through and I got a solution, which when checked seemed to work. Finally I thought, and got back to my notes.

However after a while I start to think, surely I couldn’t have been that lucky, I’d had to make about 3 guesses for it to drop out. So I tried drawing out another grid, and this time made a few different guesses. I followed it through and got another different solution. And then another one. I guess it is non-unqiue then.

Posted below are 3 of my solutions, along with the furthest I managed to get following it through logically. The numbers in black are those initially given, the numbers in red the ones added in.

Sort it out the Boar. I need Sudoku’s I can solve quickly so I don’t spend ages fiddling around finding numerous solutions and creating blog entries about it when I should be working!

- One comment Not publicly viewable

  1. Aha, someone else having trouble with the same Sudoku. I myself tried this the first week it appeared in the boar, got to a certain point with logic (not as far as you did) and then stared at it for about 5 hours trying to figure it out. After a while I decided, OK, let’s stick this into a computer and see what it says. So I googled and came across http://www.scanraid.com/sudoku.htm which solves it one step at a time—great, so I can see how it solves it. But it couldn’t. It tried all its techniques and it gets as far as you did. Then I saw the “Solution Count” button. I hit it, and to my surprise it found 8. At this point I was about ready to kill the Boar for printing a Sudoku with multiple solutions.

    Like many Sudokus, the Boar’s are rotationally symmetric; that is, the boxes in which the printed clues appear can be rotated 180 degrees without changing them; for example, the 9-2-6-3 in the bottom right is matched by 7-9-6-1 in the top left. Except for one: this one is missing a number! In the sixth box down in the first column, there is a 9 printed. So there should be a clue in the fourth box down in the ninth column. But there isn’t! The Boar left out a number when they printed this puzzle, and hence made it impossible!

    Now, this box can be a 1, a 4, or a 7. Thanks to http://www.scanraid.com/sudoku.htm, I discovered that if it’s a 4 or a 7, there are still multiple solutions. BUT, if we make the box a 1, then the puzzle does indeed have a unique solution. I assume this is what the Boar intended to include in this box, so if you want to try, fill in a 1 in the fourth box down on the ninth column and go nuts!

    05 Feb 2007, 02:27

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