Karin, you are right about the numbers in the "ears" having to be the same in a circle. However, it does not have to be a rule, since logic proves that they all must have the same number in them. Try putting two different numbers in the four ears and soon you will see that one or more of the jigsaw pieces has a duplicate number. Personally, I think this is a great puzzle for a competition as it really makes one have to think laterally. Also keeps the comments brewing up.

Guy, Udon Thani, Thailand

Looking again at the supposedly unique solution published, as compared to the alternate solution (given the rounded numbers rule), there appears to be another rule missing.

If you follow the rounded 'ears' in a clockwise circle, those ears must contain the same number. This would apply to the 4 grids in the upper left (all 3) and also for the 4 boxes in the lower right (all 9). Without this unstated rule, there are indeed 2 solutions.

Perhaps this is an argument for using only the basic Su Doku puzzle in contests? :-)

Karin Edwards, London, UK

There are two solutions even with the 'round' numbers rule. Row 4 can read 978435612 or 678495312 with row 6 being either 614792385 or 914732685. In both solution there is a round number in both round cells of the central 3x3 block, either a 9 or a 3.

Tim Locke, Sheffield,

I initially thought the puzzle did not have a uniques solution. It was only after I reviewed the "ears" of the jigsaw pieces that the light finally dawned. The "ears" are in two groups - those four in the top left and the four in the bottom right. Each group has to contain the same number otherwise duplicate numbers will occur in the jigsaw pieces. I then went on to re-solve the puzzle but without using the rule about "rounded" numbers in the round ears. It is possible, but elevates the puzzle to a higher level of difficulty.

Guy, Udon Thani, Thailand

Dear Sir,
I refer to your jigsaw soduko puzzle printed in the times on 2.4.07. which Mr. Snyder solved in five minutes. Can you please confirm that there is more than one solution, as I completed the puzzle with the 3 and 9 in the centre column reversed, which obviously changes quite a few other numbers
Even allowing for the fact that only "round" numbers were placed in the circles provided.
Kindly correct me if I am wrong, as your website insists that there is only one solution.
Yours sincerely,
Hugh (Monte)
Row 4 is 678495312
Row 6 is 914732685

Hugh Monte-D'Cruz, Hayes, U.K

Many thanks Wei-Hwa! Now that I just read your comment, it becomes fairly obvious. I need to retry it with the 'jigsaw' shapes constraint instead of the 3*3... Silly me!

Nicolas, Malmö, Sweden

There's no error in the puzzle -- I should know because I redid this image for the Times, as the original PDFs were available until late Monday :-) Nicolas and Paul, you are probably forgetting the constraint that the regions are no longer 3x3 blocks, but rather "jigsaw-shaped"

As it turns out, you don't actually need the rule that the rounded cell numbers are "roundy numbers" (3, 6, 8, 9) to solve the puzzle -- but you probably do need that rule to solve it in 5 minutes!

Wei-Hwa, Mountain View, USA

In the middle column, if you switched the 3 to the sixth row, it would now share the same nonet as the 3 in row 7, column 6. The 9 going to row 4 would cause the same problems. So, for those few placements, the "jigsaw" shapes are very important and define the unique solution.

Thomas Snyder, Cambridge, MA, USA

There's a rule missing here. Take a look at

http://wpc.puzzles.com/images/2007/wsc/jigsaw-puzzle.gif

for the missing rule (namely, the rounded cells need to have "round" numbers drawn with curved lines: 3, 6, 8, or 9)

Jason Dyer, Tucson, AZ, USA

I doubt that Mr. Snyder works on "sub straights"....they would be crooked wouldn't they?
The word is "substrates"1

Tom Dobson, Smith River, California/USA

"Can you beat 5-minute Su Doku winner?"
Probably not, but I can spell "substrates" correctly.

John, Eastbourne,

I suspect Paul, London is trying to solve this as a regular su doku, when it's actually a slightly irregular one with jigsaw piece shapes instead of 3x3 boxes. If you switch the 3 and 9 in the middle column as you suggest, then the centre bottom "jigsaw piece" contains two 3s and no 9.

Having said that, there is one key constraint on this puzzle which I can't see reproduced here, which is that the circular "bulges" on the jigsaw pieces can only contain 3, 6, 8 or 9. It may be that there are multiple solutions if that rule is omitted.

Nick Deller, Huntingdon,

There's a rule missing: the round parts have to have "round" numbers (that can only be written with curved lines) so they can only be 3, 6, 8, or 9.

Jason Dyer, Tucson, AZ, USA

The rules of the jigsaw puzzle have been omitted from the article. Each row, column and jigsaw piece has to contain the digits 1-9. (Ignore the usual 3x3 boxes during solving.) In addition, the rounded cells can only contain the "round" digits i.e. 3,6,8 or 9.

David, Belfast,

Actually the Times left out a very important rule from the original puzzle (got it on the American website of the championships): 'Additionally, the digits in the rounded cells must be "round" numbers (3,6,8 or 9)'. There you go!

Nicolas, Malmö, Sweden

This is an invalid Su Doku: there is more than one solution. After pulling my hair out for a while I decided to get some help (sad, I know) from my favourite Su Doku assistant/solver that told me right off the bat there are multiple solutions to this puzzle. Who is going to pay for my new hair implants now?

Nicolas, Malmö, Sweden

Doesn't this have two possible solutions? The 6 and 9 in column 1, the 3 and 9 in the middle column and the 6 and (in the 7th column can each go in one of two places and this will still complete properly

Paul, London,