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Posted by u/jons2cool
[response by another user]
jons2cool 1 point [ https://www.reddit.com/r/sudoku/comments/c0g0a0/could_use_a_hint/eri2y3k/June 18, 2019 20:06 GMT] (edited same day)
I sat down with a cup of coffee and finished this maelstrom today. I used a AIC Type II, which is basically just an x-chain where I used strong and weak links within a cell. After that, I used an x-wing and a naked pair and the puzzle cracked from there. It's odd how solvers don't include the logic of Type I & II AICs, Type II, especially because xy-chains can find Type I. This Type II cracked the puzzle with fewer steps than the solver. Although, when I find a Type I or Type II AIC, there are nice loops or cycles that I just didn't spot. But as a human solving the puzzle, utilizing Type I & II AICs are very useful for cracking tough puzzles.
Edit: Here's the Chain: R3c1(6)-R3c1(5)=R3c6(5)-R8c6(5)=R8c6(4)-R8c8(4)=R5c8(4)-R5c8(2)=[R5c9|R6c9](2)-R3c9(2). I had to group the last set of 2s to complete the chain. So that means that a 2 can't be in R3c1 and a 6 can't be in R3c9. Which opens up a 29 naked pair that gives some singles.
[response by another user]
jons2cool June 14, 2019 17:60 GMT
I see the 2-string kite, I already made the finned x-wing elimination with a hidden UR. I don't know what a sue de coq is and I don't think I ever would have spotted that AIC haha.
Edit: I went to add this elimination in my puzzle and I had already eliminated the 2 in r7c8 with a finned x-wing earlier but didn't update that in this post. So I guess it's just straight to the AIC
Abdlomax Jun 14, 2019 11:34 GMT
Okay, I solved this using Simultaneous Bivalue Nishio. (There must be another name for this, but it's not quite what I've seen in the books, they don't tallk about intersections and mutual confirmations.) There are lots of pairs to choose from. I settled on circling 3 at r6c9. This led to an impasse, and I don't like staring at a puzzle looking for the breakthrough. So I triangled the 2 there and this eventually led to a contradiction, so I filled in the 3 and all its consequent resolutions. That led to an impasse as before. So I chose the 3 already triangled in r9c8 [corrected from earlier error] , put a little check mark next to it, and verified the existing triangles, creating new ones, until the puzzle was completely resolved. To be complete, I would then circle the 6 in r9c8 [corrected from earlier error] and prove that this led to a contradiction. I'll do that later.
The key in using SBN is finding a good candidate pair. I looked at a few before settling on what I describe above. If I'm not careful and pick a punk pair, I end up with a confusing mess -- remember, I'm working in ink. I like the clarity of ink, it is easier to see than pencil marks. But don't make any mistakes! I can recover from some mistakes, but it becomes quite a mess.
I can upload my worksheet if anyone wants to see it. [done, see link below.][Copy on the right]
note added: this was a Maelstrom-level puzzle. It was easy with SBN. The "guesswork" -- or skill -- is in choosing a decent starting pair. I did not look for the "best." Only for one good enough. As we will see below, there are puzzles where there are very few such options, and they still leave the puzzle very complex. But I have not encountered, yet, such puzzles in print. It appears they are deliberately avoided. That is, there is an opinion that there are Sudoku where you must guess, and this is not considered a "good Sudoku," by Peter Gordon. (Mensa Guide to Solving Sudoku, p. 86.) SBN is not guessing. Nor is it "trial and error.) It is the investigation of two opposing sets of resolutions, and seeing how they interact. In any case, neither is Ariadne's thread "guessing;" rather, it is following an algorithm that must resolve any Sudoku, if there is a solution. If choosing a pair to investigate is "guessing," so is choosing to look for any of the very many advanced strategies, where most of these guesses are useless most of the time. Nevertheless, SBN, precisely because it is easy, once understood, may not be as fun and finding that spectacularly complicated strategy that unlocks the puzzle. That is a personal choice.
I would love to see a Sudoku book that announced, at the beginning, "All the puzzles in this book have a unique solution and no other solutions, except for three. One has no solution, the other has two. And then at the end, "These three puzzles are not solveable by any of the strategies [with reference to a list of advanced strategies, perhaps excluding Bowman's Bingo, which is like SBN in some ways}, consider this a Project Sudoku. Solving it will require a truly advanced technique. One of these puzzles has two solutions. Obviously, Ariadne's Thread can solve these, because it can solve any Sudoku. It will find a solution, but will not generally guarantee that it is unique. Can you find a more efficient way? See our page on this puzzle, and the public comments." --Abd (talk) 13:36, 24 June 2019 (UTC)