dailysudoku.com

Steve R · Posted: Tue Jan 17, 2006 2:51 pm Post subject: Nightmare

Masochists participating in this forum (and there are a few) might be interested in Ruud van der Werf’s site Sudo Cue. His daily “Nightmare” puzzles are meant for you. If one is neither fish nor fowl, the chains are needed. Today’s (17 January) is not especially difficult by Ruud’s standards but it offers a flavour and you can always try 4 January if it merely serves to whet your appetite.

Sleep well!

Steve

Steve R · Posted: Tue Jan 17, 2006 3:05 pm Post subject: PS

I expected the link to appear. Perhaps it will now.

Steve

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

Thanks for the link, Steve.

I haven't tried the 4-Jan puzzle yet, but I did take a stab at the one for today. It was pretty simple to get through -- if I assumed the existence of a unique solution. In fact, I managed to use that assumption twice -- on two different "Type-4B Unique Rectangles" (a formation that Keith brought to our attention some time ago). dcb

Steve R · Posted: Wed Jan 18, 2006 12:03 am Post subject: Nightmare

Yes. An X-Wing will also do it, without further assumption.
I was unable to solve the fourth of January. Puzzle, which made me think it might be a little harder.

Steve

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

I must be a masochist, Steve. :)

From the initial "Nightmare" puzzle for Jan 4, 2006

Steve R · Posted: Wed Jan 18, 2006 8:57 pm Post subject: Jan 4 "Nightmare"

That’s a neat solution, David.

I stalled a little before the stage shown in your grid. It was only when going through your analysis that I found some wings to get me there. I have since tried Sudo Cue’s solver on the puzzle. The analyser started in the obvious way, found the pair (38) in box (2, 1) and then calmly announced: “R6C5 has Digit 8 as the only remaining candidate.” How it managed to exclude the 3 is beyond me but I have not seen the solver before so there is probably more to it somewhere. In the meantime I shall stick with the solution I can understand. Good stuff!

Steve

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

The "Nightmare" site is a lot of fun. This puzzle is for Thursday, January 19.

someone_somewhere · Joined: 07 Aug 2005 Posts: 275 Location: Munich

Hi,

I found a Hidden Triple 5 7 8 in r1c8 r2c7 r2c8
and could eliminate 6 from r1c8, r2c7, r2c8.

Sometimes later, I found Hidden Pair 3 8 in r8c3 and r9c2 (in 3x3 Block)
and could eliminate it from r8c3, r8c3.
And then 6 not in r8c1, Nacked Pair 4 6 in r7c3 and r9c1 (same 3x3 Block).

Than I started the "double implication chains from pairs":
6 = r5c3, 6 <> r5c8, 6 = r9c8
6 = r5c3, 6 <> r7c3, 6 = r9c1
which leads to a contradiction for 6 in row 9, concluding:
6 not in r5c3.

6 = r3c7, 6 <> r4c7, 6 <> r5c7, 6<> r6c7, 6 = r5c8
6 = r5c7, 6 <> r3c6, 6 = r5c6
which leads to a contradiction for 6 in row 5, concluding:
6 not in r3c7

A little bit later I found a nice Swordfish on Row for digit 6:
in r6c3 r6c7 r7c3 r7c4 r8c4 r8c7 eliminating it from:
r2c3, r4c3, r4c7, r5c7, r9c7 and r9c4.

and finally a nice XY-wing: X=4 Y=9 in r2c3 X=4 Z=6 in r2c5 Y=9 Z=6 in r3c2 leads me to eliminate 6 from r3c6.

I think that is pretty close to David's solution. I did not count the remaining 6's but the the puzzle is solved after dealing with them.

see u,

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

someone_somewhere · Joined: 07 Aug 2005 Posts: 275 Location: Munich

Hi,

The distribution of 6's is relly BEAUTIFULL.
If we eliminate first the one from r2c9 we are left with exact 2 occurences of digit 6 in every block!

It would be a pitty to distroy it with the XY-wing, as you pointed.

I am using seldom coloring, as I worked a lot with it and it does not help in a lot of puzzles. So, I found a nice excuse for my "color-blindness".

I prefer now the "double implication chains" that are almost always giving good results. For example, in this case, a "5 star constallation" of 6's with the alpha star in r4c6 and the r4c2, r3c2 and r5c6, r3c6 is giving a contradiction in row 3, so that the aplha could be exploded.
This is equivalent to the coloring for a subset of nodes that is minimal for getting a contradiction.

This is one of the nice coloring examples that I ever have seen.

And it starts with 19 elements in the exclude table. My previous finding was that situations with 17 or 19 elements have the highest chance to give results with the coloring thechnique.

see u,