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Steve R · Posted: Sat Feb 09, 2008 8:18 pm Post subject:

If you do not assume the puzzle has a unique solution, box 9 is of interest.

The (19) in r4c8 means that r7c8 and r8c8 cannot contain both 1 and 9: between them they must contain a 2 or a 5. Bearing in mind the (25) in r8c9, 2 may be eliminated from r8c3. This may be the colouring you noted.

Of itself the elimination does not solve the puzzle. However, the rest is relatively straightforward.

Steve

Asellus · Posted: Sat Feb 09, 2008 10:37 pm Post subject:

Steve,

It seems to me that what you have done might be considered APE.

However, there is also a Sue de Coq. The 4 cells r478c8 and r8c9 contain the four digits {1259}. {19} in r4c8 shares c8 with r78c8 and {25} in r8c9 shares b9 with r78c8. Thus, <2> can be removed from r7c9 and <5> can be removed from r7c7 and r7c9. This creates a Skyscraper on <2> in r17 (eliminating that <2> in r7c3).

The coloring mentioned above is probably the same as the Finned X Wing elimination of <2> from r7c8. If this is done, then the Skyscraper isn't necessary.

After all this, a single XY Wing solves the puzzle.