dailysudoku.com

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

Since the Aug 22 VH was solved too quickly, I went looking for a challenge and promptly got my self stuck. Help!

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

There's a strong link exclusion for r1c1.

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

Earl,

Take a look in C9 for Tracy's strong link exclusion.

Asellus · Posted: Wed Aug 22, 2007 9:51 pm Post subject:

You can make the same elimination with a simple finned fish, also involving C9. Plus, there is an XYZ-Wing in the lower right region of the grid.

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

I see the finned fish, a bit of a variation.
I see the xyz wing, but not that it eliminates anything.
I see the strong link in C9 and box 7, but I do not see how they are strongly linked, unless you can eliminate the 5's from R7C7,8.

Thanks for the help.

Earl

Mogulmeister · Joined: 03 May 2007 Posts: 1151

Earl,

When 2 strong links intersect (share) a house, then if the ends (or pincers) can both see a candidate it can be eliminated. In this case the pincers are r1c9 and r9c1.

Look at the diagrams below.

The 2 strong links on 5 are in box 7 and column 9. They intersect on row 7. Lets say green is true and orange is false.

The only outcome that would cause a failure is if r9c1 and r1c9 were both false at the same time. However, as you can see - set either pincer to false and the other pincer is always true.

The 5 in r1c1 can't survive.

Marty R. · Posted: Thu Aug 23, 2007 4:06 am Post subject:

I'll take the simple Finned X-Wing in columns 3 and 9, since I'd have found that before the strong links, indeed, if I ever would have found them.

Asellus · Posted: Thu Aug 23, 2007 7:00 am Post subject:

This strong link type of elimination is another "Color Wing" example. R7 provides the weak link "Bridge". Some brief reflection shows that it is essentially the same as a Skyscraper or a Kite or Turbot Fish in its logical structure, which is why I find the more general notion of a "Color Wing" to be so useful.

All of these named "sub-types" often have the structure of two 2-cell strong links joined together by a weak link.

Examining a puzzle for the locations of strong links is often helpful in finding solutions, in my experience. They may lead to useful simple coloring or a simple "Color Wing" (aka multi-coloring). If the links are not extensive, then the coloring is not difficult to do in ones head. (After all, who needs to color their Skyscrapers and Kites?) That is a boon for pencil and paper types.

It seems a shame to be able to spot Skyscrapers but not be able to spot the logically identical and no more complex structure discussed above.

PS: While it is true that it if one of the "pincers" is false then the other must be true (and this fact alone is enough to justify the elimination), it is also true that it is possible for both "pincers" to be true. (To see this, suppose that R7C8 is the correct <5> cell.) This is a consequence of the weak link.

Asellus · Posted: Thu Aug 23, 2007 7:05 am Post subject:

Mogulmeister · Joined: 03 May 2007 Posts: 1151

I think more formally this structure is known as an "X-cycle".

Hasel · Joined: 23 Aug 2007 Posts: 4

I find it difficult ......

Hasel
http://classicalpearls.blog.com/

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

Thanks to all of you for this discussion of strong pairs, weakly linked.
It is a new insight for me.

I have avoided "multiple coloring" since it seemed so complex - no fun. Strong pairs, weakly linked, I can handle.

Earl