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Nightmare November 9th
Posted: Fri Nov 09, 2007 10:57 pm
by Para
Hi Ruud
I know there usually isn't much feedback from the nightmare puzzles. I just wanted to say that todays puzzle was a real beauty. Enjoyed solving it. Besides the standard sudoku techniques i used 2 y-wings and 1 xy-wing to open up the puzzle. Finally a BUG+1 to crack it.
greetings
Para
Posted: Tue Nov 13, 2007 2:34 am
by Sudtyro
Hi Para,
Could you please clarify some nomenclature? I'm confused about exactly what a "y-wing" is. Sudopedia redirects to xy-wing, and Andrew Stuart's Sudoku Solver site says "aka xy-wing." But you refer separately to both.
TIA!
Posted: Tue Nov 13, 2007 8:37 am
by Para
Hi
Here's a link to something udosuk wrote about y-wings, xy-wings and xyz-wings.
http://www.djape.net/sudoku/forum/viewtopic.php?t=963
Think that will be clear enough.
If you want i can show you the steps i did for this puzzle.
greetings
Para
ps. officially one of the y-wings, should have been termed as a grouped y-wing.
Y-Wing
Posted: Tue Nov 13, 2007 5:11 pm
by Sudtyro
Mystery solved! Thanks for the link to udosuk's writeup.
It's interesting...sometime back I ran across your post
here and saved a hardcopy for my "wings" folder because it seemed like such a easy pattern to search for. It didn't seem to have a name, though, and I just couldn't imagine Y-not!
[Hint for Ruud] It would be really nice to see an article (or at least a link) added to Sudopedia!
Posted: Tue Nov 13, 2007 5:54 pm
by Para
Hi
By the time i wrote that message i didn't know it's name either. I once wrote out a bit in the sudoku.com forum too (
http://www.sudoku.com/boards/viewtopic. ... 2236#42236), mostly about the further implications of this technique. There no-one could inform me about the name of the technique either (or no-one felt like informing me on it) but after seeing that post by udosuk i found a name for it. It seems that this technique is known under multiple names. I think it is also known as remote pair exclusion or w-wing. But i guess that is a general problem in the sudoku community.
But i find this technique easy to search for. Mostly because i have certain places in the grid where they might appear(either by checking common bivalue cells or checking for only 2 options for one digit in a house). I am able to find ALS-y-wings easier than normal ALS-xz eliminations for example.
Well i hope you're able to use the y-wing now.
greetings
Para
Posted: Wed Nov 14, 2007 11:07 am
by rep'nA
Here is a little history that I wrote up for Keith on a different forum
I'll try to give you the history as I know it. The first recorded use of a w-wing that I know of is John Macleod in Jan. 2006, who called it the Corkscrew Rule:
http://www.sudoku.org.uk/cgi-bin/discus ... 0#POST2790
His rule was overly restrictive and I pointed this out formulating the general rule. I subsequently forgot about this exchange and didn't check for these things.
In March of this year, Stephen K posted his
Y-wing styles thread which used the w-wing (not called that) as a main example of his more general notion. I posted often in this thread and suggested that the specific technique be called a semi-remote naked pair. This same has become semi-standard (in the sense that people usually call the technique several names when they use it and semi-remote naked pair is one of these names).
At some point in August, [Keith] created the thread on
W-wings and gave some very cool corollaries of the rule.
So, in principle, the w-wing/semi-remote naked pair/y-wing style are all the same. In practice, I've only seen w-wings used in the way xy-wings are used, while I use semi-remote naked pairs allowing for longer chains in between the endpoint pairs.
By the way, I recently posted what I think is a rather slick application of semi-remote naked pairs in a solution to one of
wapati's puzzles.