dailysudoku.com

[lkmckin]

Hi, new here. I've been trying all the VH puzzles on the site, and have gotten pretty good at X-wing, XY-wing, swordfish, etc.

But todays LA Times puzzle is a real stumper. For starters, the grade given to it by this site's "play on line" function is TOO HARD, even after inputting the puzzle raw. I've worked up to the following, still rated TOO HARD. Can one of the real experts give me a clue as to how to proceed (other than trial and error, I hate that).

Thanks in advance.

Lindsay

[Nwohio]

I'm no expert, but take a look at r6c7c8 and r8c7c8.

[Mogulmeister]

Look for a unique rectangle on <59> I'm not mad about 'em but it sure works.

[Ruud]

If you do not fancy unique rectangles (who doesn't?), there is a nice 2SDG (Sue de Coq) alternative:

r56c8 has candidates {4,5,8,9}
because r4c8 = {4,8}, r56c8 must contain either 5 or 9.
because r8c7 = {5,9}, r56c8 must contain either 4 or 8.

so: r56c8 contains {5 or 9} + {4 or 8}

as a result, you have a naked pair {4,8} in box 6, and a naked pair {5,9} in column 7.

Ruud

[Mogulmeister]

Nice one Ruud - do you see lots of these ? It's a new one on me - looks like a multi-candidates ALS.

[lkmckin]

Very interesting. Kind of an bifurcation of naked pairs. Where does the term 2SDG(Q?) Sue de Coq come from?

Of note is that the puzzle grade goes from "too hard" to "easy" after the application of this logic.

Thanks, Ruud (and Mogulmeister without whom I never would have understood).

[Ruud]

Thanks Mogulmeister for the correction.

I found another one in the recent Andrew Stuart puzzle, but that does not really advance the puzzle. These patterns occur frequently in Sudoku puzzles, but because of their complexity, they are not easy to spot.

2SDG means 2-Sector Disjoint Groups/Subsets, with Disjoint Group/Subset being the early name for naked/hidden subset. The technique was found in 2005 by Rubylips, one of the early sudoku programmers. At that time, he was using the nickname Sue De Coq on the Pappocom forum and the technique was soon named after its inventor.

There is another example in Sudopedia, and there is an interesting discussion on another forum.

Ruud

[Mogulmeister]

Cheers Ruud - always nice to continue learning!