Done

hard one

a very hard medium

hi all

not too bad

challenging for me, but I got it after a couple of guesses.

Difficulty score 30. Requires cross hatching, missing digit scans and region intersection analysis (scanraid calls it pointing pairs).

are there really strategies and terms for a puzzle?

Yes. An elementary introduction can be found at Wikipedia. A more advanced treatment is at http://www.scanraid.com/sudoku.htm.

Done!

Saratara question is a good one, because unfortunately there is no universal agreement on terms for strategies and other concepts related to Sudoku.

I define region much like the Wikipedia article on Sudoku; the 3x3 squares that must contain one of each of the 9 digits.

However, I think it is useful to also use the term region a little more loosely to also refer to the rows and columns.

Hence, region interesection is looking at the place where a 3x3 region and a row or column intersect.

If a particular digit can only appear in 2 or 3 empty cells in a row and those empty cells all lie in the same 3x3 region, then that digit can be eliminated from all other cells in the 3x3 region.

It works for columns and 3x3 regions, and you can reverse the logic to eliminate the possibility of a digit appearing in the row or column outside of an intersecting 3x3 region.

I use cross hatching as defined in the Wikipedia article on Sudoku. Wikipedia has pictures and once you see it I think you will agree it is the most descriptive term to describe a techique everone had to figure out before they could solve their first puzzle.

A missing digit scan is when you consider an empty square by tabulating all the digits used in the row, column and region containing the cell. If 8 of the 9 digits are present, then the missing digit MUST go in your empty cell.

I don't remember where I saw it, but I agree with the idea that the word "square" should not be used in relation to Sudoku, it's too ambiguous.

I usually use the word "block" to refer to the 3x3 areas.

I like it. Maybe I will too. We could reserve regions to refer to rows, column or blocks collectively.

a note on the missing digit scan: sometimes you only have 5 or 6 of the 9 filled in (say everything but 4,6,7 and 9), but two of them must be, say, 6 and 7, because they're both the only missing digits in an intersecting row (, column, or block) and of the two remaining, one *can't* be a 4, so the other *has to* be a 9.

I think you are referring to the Naked Pairs technique (see http://www.scanraid.com/sudoku.htm).

Or more accurately, you seem to be describing an application of Nake Pairs analysis followed by a cross hatching scan.

Yay I'm pro.

congrats!

congrats

Thanks :) Hmm, no Icelandic flag?

That was the number one reason for registering :P

Why don't you email Dr God - maybe he can add it for you

good idea.. right after I finish this puzzle, hehe

Rats, I made some mistake and I don't have any checkpoint.

Rats, I didn't recover from that mistake after all. I thought I was doing okay.

Nuts, I think I'll quit.

done without greens or checkpoints - nice puzzle though, harder I think than some of the recent "hards"

I can't believe it, I forgot to do the sudoku yesterday, and it was a hard too. *cries* :(

don't worry aki, this one is more fun anyway :)

(he says, 2 hours later)

Hi!

hey

in

done!

hey look, i keep forgetting to finish this

stupid work, getting in the way of sudoku

hi all

how goes?

pretty decent

you?

frazzled

but good :)

there we go... off to archives now

yeah, i've got all of thanksgiving vacation to go back and finish

that will have to wait until i'm less busy

I just upgraded to pro... I've got 8 still to finish in march, and then most of the puzzles since then

I nearly gave up on March 28th's

still haven't tried that one.. got something to look forward to then :P

some solvers can't break it

I'll be resorting to some brute force

crap

finally

Finally. That was a little tough for a "medium"

go

done

go

go

pause

unpause

done!

That was tough