dailysudoku.com

nataraj · Posted: Wed Oct 03, 2007 6:34 am Post subject: oct 3 vh

so many cells with {1,8} ... calls for a w-wing:

one of r5c2 and r6c4 must be 8,
(connected by: 52( 18 )-42( 18 )-48( 28 )-66( 18 )-64( 18 ) )
so remove 8 from r5c5 and get naked pair 12 in box 5, thus r6c4=8

Later, remove 3 from r8c3 by this chain:

85-94=97-78=18-14=25-85

and the puzze is solved.

I am sure there were many more wings ...

George Woods · Joined: 28 Mar 2006 Posts: 304 Location: Dorset UK

sdq_pete · Joined: 30 Apr 2007 Posts: 119 Location: Rotterdam, NL

I reached a very similar position, I guess, and also had to resort to "non-PC" logic:

Johan · Joined: 25 Jun 2007 Posts: 206 Location: Bornem Belgium

sdq_pete · Joined: 30 Apr 2007 Posts: 119 Location: Rotterdam, NL

Oh, I had in fact seen the X-wing on 4's but still needed the logic I mentioned. I shall look if I can find the XY-wing.

Peter

TexCat · Joined: 07 Jul 2006 Posts: 32

I used a skyscraper of 3's. Or perhaps it was several skyscrapers of 3's. : - )

Earl · Joined: 30 May 2007 Posts: 677 Location: Victoria, KS

An x-wing to eliminate a 4, an x-y wing to eliminate a 1, and an xyz-wing to eliminate a 6, and I was home.

Earl

sheryl · Joined: 24 Sep 2007 Posts: 64 Location: New York

can you please explain what this chain is?

Later, remove 3 from r8c3 by this chain:

85-94=97-78=18-14=25-85

can you explain what it is you are doing here. i'm down to these last few numbers and cannot find a way to eliminate them. i'm fairly new and i don't know what a wing is.

thanks.

nataraj · Posted: Wed Oct 03, 2007 3:51 pm Post subject:

sure I'll explain.

the numbers in the chain are coordinates of cells containing 3, "85" means r8c5.

an equal sign (=) means there are exactly 2 such cells in the unit that both cells share (94=97 means there is a 3 in row 9 in exactly two columns 4 and 7. The implication of this "strong link" being that if one cell is NOT 3, then the other is).

a dash (-) means the two cells share a unit (implication of this "weak link": if one cell is 3, the other cannot be 3)

the whole chain then reads like this: if r8c5 is 3 then r9c4 cannot be 3 then .. r8c5 is NOT 3. A contradiction, therefore r8c5 cannot be 3.

And of course there's a typo in my post: 3 is removed from r8c5 not r8c3

In practice I usually draw little diagrams of dots (one for each digit) and only connect the strong links. The chains are very easy to spot in these diagrams (see picture here:)

http://www.pictureupload.de/pictures/031007181032_Image014.gif

You'll find this technique under "x-chain" in sudopedia:
http://www.sudopedia.org/wiki/X-Chain

sheryl · Joined: 24 Sep 2007 Posts: 64 Location: New York

i wish i could say i understand, but i really am not getting it.

the whole chain then reads like this: if r8c5 is 3 then r9c4 cannot be 3 then .. r8c5 is NOT 3. A contradiction, therefore r8c5 cannot be 3.

i don't understand how you cme to the conclusion that 85 is not a 3. why can't 85 be a 3 and 94 not be the 3?

i think if i understood that it might help. i've looked at your diagram which i don't understand and i've looked at sudopedia but following the logic without understanding the terms yet is a bit difficult.

if you think i'm asking too much, i will understand. i appreciate your trying to help. thanks.

duffy · Joined: 13 Sep 2007 Posts: 26 Location: Toronto Canada

Sheryl,
I like x-, xy- and xyz-wings, but they can be relatively difficult to spot: it is worth reading up on them. (Chains and W-wings are not part of my normal pencil and paper approach.) In this case, I probably did what Earl did. Maybe the following grid will help illustrate the use of these wings in this particular puzzle.

Clement · Posted: Wed Oct 03, 2007 6:04 pm Post subject: Daily Sudoku Oct 3-2007

An XY-Wing with a pivot at r4c8{2,8} eliminating 1 at r6c4.After that the puzzle is solved by normal elimination.

sheryl · Joined: 24 Sep 2007 Posts: 64 Location: New York

duffy,
i appreciate the help. is there a particular book that you would suggest? i've read several different books, including an explanation in sudopedia, but i need the basics explained, e.g. what is an x wing or a y wing .. when the terms are used without the basic explanation it is just not understandable to me. i will study your diagram some more, but so far, it is not becoming clear to me. when you learned this, did you actually learn it from a book or did someone explain it to you? i think if someone actually showed me rather than tried to tell me i would be able to pick it up. i'm going to have a friend of mine sign up and read you explanations, maybe he can explain it to me.

anyway, i very much appreciate your trying and i will keep looking at your diagram, something might gel!

Marty R. · Posted: Wed Oct 03, 2007 6:22 pm Post subject:

Sheryl, if Sudopedia isn't helpful, try searching for the specific technique you're interested in. For example, search for "sudoku x-wing" (without the quote marks) and you'll have plenty of choices, increasing the odds that you'll find the type of explanation that suits you.

duffy · Joined: 13 Sep 2007 Posts: 26 Location: Toronto Canada

Sheryl,
I haven't used any books, just lots of practice and follow-up on web entries. I have found, on this site, lots of good explanations by Marty and Keith, in particular. Also, Marty's search suggestion is a good one; a particular site with good explanations that I used recently is brainbashers.com. And remember, once again, that one normally won't be able to absorb too many complex descriptions at a time.
Idea

Don.

sheryl · Joined: 24 Sep 2007 Posts: 64 Location: New York

thanks everyone. i'll let you know how i do!

Steve R · Posted: Wed Oct 03, 2007 7:23 pm Post subject:

As a matter of interest the BUG+3 pattern also eliminates 4 from r8c4.

Suppose you write the George and Peter’s grid in this form

cgordon · Joined: 04 May 2007 Posts: 769 Location: ontario, canada

keith · Posted: Thu Oct 04, 2007 1:55 am Post subject:

Given the techniques that the "very hard" puzzles may require, this one can be solved with an X-wing, and XY-wing, and an XYZ-wing.

After the first two, I get to here: