dailysudoku.com

DanPMK · Joined: 29 May 2006 Posts: 2

Ok, I'm semi-new to this puzzle, but I think I got it down, when I run across this. I print this one out, and I get 6 numbers in. Then I get stuck. So, I look at every possible number. Still nothing definitive. So I erase and try again. I get the exact same problem. I go to the Draw/Play, and it gives me the next solution, but I think it's either some error or something I'm missing, as it doesn't seem like a logical step. Here's what I have so far:

David Bryant · Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

Hi, DanPMK! Welcome to the forum.

The short answer is that there's an X-Wing formation in rows 1 and 9 that eliminates the possibility of an "8" at r4c9. Let me explain.

-- Notice the "8" at r6c4. Because of its position, we can see that the "8" in the middle left 3x3 box must lie in column 3.

-- This fact, together with the "8"s in columns 4 and 5, implies that the only two places an "8" can fit in row 1 is at r1c1 or r1c9.

-- We have a similar situation in row 9, where the only place for an "8" is at r9c1, or r9c9.

-- So there's either an "8"at r1c1, and at r9c9. or else there's an "8" at r9c1 and at r1c9. Either way there cannot be an "8" at r4c9. dcb

DanPMK · Joined: 29 May 2006 Posts: 2

I see, I've never had to use the more complex logical rules when solving a puzzle. I'll have to look into them. Thanks!

Marty R. · Posted: Tue May 30, 2006 1:07 am Post subject:

After the basic stuff to eliminate candidates, I used that X-Wing that David talked about. Later, seeing nothing else available, I tried a forcing chain from which one value resulted in a contradiction. Finally, I saw a rectangle of the ab-ab-abc-abd type, with the two ab's on a diagonal. I tested for c and d, and one of the values opened up the whole puzzle.

I don't know anything about Draw/Play, I'm strictly a paper-and-pencil guy. I found this puzzle a little more difficult than others with the same rating.

David Bryant · Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

You're right, Marty -- this is probably the toughest DailySudoku puzzle yet.

After following up on the first X-Wing (and locating a "4" at r2c3) a second X-Wing emerges -- on "6"s, in columns 3 & 4. I had to search pretty hard to find it. dcb

Huw Llewellyn · Joined: 30 May 2006 Posts: 1 Location: Canberra Australia

Thanks for the tips on this one- definitely the Sudoku from hell!
I have yet to see one as hard as this. Once you pointed out the second X wing I was on my way. A good one to learn advanced techniques on.

kkklau · Joined: 31 May 2006 Posts: 2

I arrived at the same method like David's. It's the first time I have to use X-wing twice. Spotting x-wing is not easy in the beginning and it is the hardest technique I can use. Beyond that I don't quite get it.

Marty R. · Posted: Wed May 31, 2006 3:46 pm Post subject:

Welcome to the forum to the previous two posters. I didn't see David's second X-Wing, but still completed the puzzle. On just about every specific puzzle I see discussed, it is apparent that there are multiple approaches that can lead to a solution, and this, obviously, was no exception.

riderdude · Joined: 04 Jun 2006 Posts: 9 Location: Mass

David Bryant · Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

Hi, riderdude! Welcome to the forum.

I'll try to explain the X-Wing logic (X-Wing on rows) one more time.

-- At the position DanPMK had reached, there are only two places to fit an "8" in the first row -- either at r1c1, or else at r1c9.

-- There are also only two places to fit an "8" in the ninth row -- either at r9c1, or else at r9c9.

-- In the final solution, there has to be exactly one "8" in the first row, and there also has to be exactly one "8" in the ninth row.

-- Therefore there are only two possible ways the digit "8" can fit in rows 1 and 9. Either r1c1 = 8 and r9c9 = 8, or else r1c9 = 8 and r9c1 = 8. Those are the only two ways we can fit an "8" in the first and ninth rows.

-- We can conclude that the "8" in column one appears either at r1c1 or at r9c1, and that the "8" in column nine appears either at r1c9 or at r9c9. So there cannot possibly be an "8" in any other cell in these two columns. dcb

riderdude · Joined: 04 Jun 2006 Posts: 9 Location: Mass

sorry. either i'm being thick or inarticulate.
or both.
i still don't see why 8s can't go in r2c1 and r7c1, knowing that the 8 in column 3 goes either in r4c3 or r5c3, and the positions of the other 2 8s. i get everything except that.

David Bryant · Joined: 29 Jul 2005 Posts: 559 Location: Denver, Colorado

riderdude · Joined: 04 Jun 2006 Posts: 9 Location: Mass

oh, ok. being thick.
got it.
thanks, and sorry.

geogecko · Joined: 04 Jul 2006 Posts: 5

TKiel · Joined: 22 Feb 2006 Posts: 292 Location: Kalamazoo, MI

Geogecko,

With the grid as you posted it, the only places for a 4 in c9 are both in box 3 (r13c9) which means all other 4's in that box can be excluded (r3c78, r2c7). This leaves r2c3 as the only place in that row for a 4.

geogecko · Joined: 04 Jul 2006 Posts: 5

Ah, I see that now. Thanks for the tip. Sometimes those are hard to find too. I like the pattern finds better, but then that's what makes them easy.

geogecko · Joined: 04 Jul 2006 Posts: 5

This one has been the hardest one I've done so far...

So, after the 4, anyone got any logic for getting the "6" in R7C7? I have a "14568" for a possibility in that cell.

keith · Posted: Wed Jul 05, 2006 12:42 am Post subject:

geogecko,

In the position you posted:

geogecko · Joined: 04 Jul 2006 Posts: 5

Do you know of a web site that explains some of the techniques you are talking about?

Thanks for the assistance!

Ah, I found your post about Unique Rectangles. Thanks!

TKiel · Joined: 22 Feb 2006 Posts: 292 Location: Kalamazoo, MI

Geogecko,

After the single in row 7 and then the box-line interaction (exclusions in box 1), as mentioned by Keith, there is an x-wing on 8. none of which help with putting a 6 in r7c7. For that you need to spot the swordfish.

For info on terminology & techniques (although there is no consensus on terminology) you can view the following links:

www.sadmansoftware.com/sudoku/techniques.htm
www.angusj.com/sudoku/hints.php
www.scanraid.com/BasicStrategies.htm