All entries for Friday 14 August 2009

August 14, 2009

Friday Puzzles #10

This week comes with a bit of a special theme. The number 10. It’s also very special in that I have managed to get this out and published on a Friday. Wow!

As regular readers may or may not know, last April I participated in the 4th World Sudoku Championships, hosted in a dusty provincial Slovakian town called Zilina. Much was made of the (lack of) quality of many of the puzzles – but one of the most enjoyable puzzles was a variant that appeared in the semi final:

Puzzle M3 – World Sudoku Championship, Zilina 2009

The idea was simple – take a classic sudoku, and mark exactly where an adjacent pair of digits summed to 10. So if a 3 and a 7 were next to each other, a cage was drawn round and marked 10. The twist with this is the contrapositive inference – that is if no cage exists round a pair of adjacent cells, then the numbers inside cannot sum to 10! The puzzle itself had only one such pair of numbers marked – which made me wonder if actually you could go one better and perfect the puzzle.

Well, it turns out you can. After a lot of painstaking work to first even find a valid solution grid, I then wrestled with the task of making a decent puzzle – with my given visual theme. The result isn’t particularly challenging – though not totally trivial.

I do also have a couple of very hard puzzles too. They turned out to be hard almost by chance – they certainly aren’t a particularly pleasant solve in my opinion and so I haven’t bothered publishing them. Anyhow – without further ado, here’s the preamble: place the digits 1-9 in each row column and marked 3×3 box. Digits in adjacent cells may not sum to 10.

#014 No Tens Sudoku – rated easy

All puzzles © Tom Collyer 2009. Well, obviously excepting the world championship puzzle above. I have no idea who to credit that to – and whilst used without permission, I’d like to think that demonstrating it would come under fair use. Anyhow, please respect the rights of the author.


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Welcome to the blog of current UKPA sudoku champion, two-time Times national sudoku champion and general logic puzzle fan Tom Collyer.



Home of the original Friday Puzzles, each Friday I publish a 100% original and handmade logic puzzle, inspired by the world-famous Nikoli company.


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Latest comments

  • I like the appearance of the solution. :P Nice one. by Prasanna Seshadri on this entry
  • I think I've seen something vaguely similar in some of Palmer's puzzles as well. To be honest I've a… by on this entry
  • That's two puzzles in a row where I find something I had used before. I know, not plagiarism; just g… by Bram on this entry
  • Kota, that's not really true. I had made my puzzle before going to the WPC already. Also I don't hav… by Bram on this entry
  • About 20 minutes, after restarting from a mistaken conclusion. by Bryce Herdt on this entry

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