Advanced Sudoku: The Finned X Wing

We solve a puzzle requested by one of our viewers and it certainly doesn't disappoint. This is an extremely hard Sudoku and, in the end, Simon resorts to the esoteric Finned X-Wing to make a crucial breakthrough.

[@victorchan6820]

Wow, there's actually 2 finned X-wings, not just one. Simon explained the one in the video, but there's another X-wing in r1c68 r7c6/b9. That eliminates the 5s in r7c79, which in turn eliminates the same 5 as the other finned X-wing. Great puzzle; everything was perfectly set up, but very difficult to find. Even the naked single in r1c2 took me forever to discover.

[@JesteR00160016]

there's 3 sahimi) 2 of them or 1 skyscraper r8c5-c8, r1c6-c8. Eliminates 5 from r3c5 and r7c6. Or simon's (we have 7 5 8 triple so r3c8 is not five , so what Simon found is sashimi x-wing. And yours (only one true "finned")

[@JesteR00160016]

For me it was much easier to spot skyscraper. Just find 2 rows with 2 possible possitions, connected by base, possible eliminations by 'towers'

[@inlumina_punctro]

@@JesteR00160016 Also his finned scheme is composed of 2 simetrical sky scraper and one x wing. :)

[@goldenera7090]

perhaps one of the difficult solves on this channel. super solving Simon. thanks for the video

[@susanlemmey4012]

The last logical step also had an empty rectangle doing the same thing as the 'finned X-wing' using most of the same squares too.

[@inlumina_punctro]

Your variant is even more logic, I mean more obvious and easy to demonstrate. Also you can imagine two simetrical sky scrapers which eliminate that five in R1C8...and of course combined with the x wing. :)

[@rabidsamfan]

That was a real challenge. An excellent solve.

[@JohnRandomness105]

Interesting. The finned X-wing that I found in 5s, that finally cracked the puzzle, had the same row-8 pair, but instead I used row 1. The 5 was constrained to the fin (column 6) and column 8. That still made it a finned X-wing, even though the 5 was barred from R1C5. EDIT: This finned X-wing is another case of a position that can be treated as both a finned X-wing and an empty rectangle. Four times, I've found a finned X-wing in a puzzle, and the video processed it as an empty rectangle. At the time of the first three, I didn't know what an empty rectangle was -- I had no choice but to process it as a finned X-wing.

[@travholt]

Hm, you already had figured out the naked triplet 578 in column 8, which eliminates the 5 in R3C8. How would that have changed your reasoning?

[@CrackingTheCryptic]

Good question and perhaps worth a short video follow up to explain. The short answer is that it doesn't change the reasoning: the interesting thing about a finned x-wing is that the final corner of the x-wing shape could even be filled by another number and the logic still works! Imagine that r3c8 actually contained a 2 at the point we get to the discussion on finned x-wings in the video: what does this mean in terms of next steps? Well, clearly if either "fin" is a 5 then we can eliminate the 5 in r1c8; what if neither fin is a 5? If neither fin is a 5, then we KNOW that r3c5 must contain a 5 because this is the only cell in r3 left that can contain a 5. This in turn means that r8c5 cannot contain a 5 which, in turn, means that the 5 in row 8 MUST be in column 8. So the upshot is that we can still remove the 5 from r1c8 in this case too. Make sense?

[@lsalawa]

I don’t see finned x wing there as c8r3 cannot be 5 as it’s eliminated by naked triple in this column. However there is a skyscraper which have the same result.

[@debralegorreta1375]

I went at it pretending to be Mark, looking for a good place to with two alternatives, one of which would create havoc on the rest of the puzzle. After the initial fills, I found that the only numbers that could go in R1C3 were 7 and 8; and that the 8 would indeed create a great deal of devastation. So I went with that hypothesis, that R1C3 is 8, to great effect. That forced twenty or so moves until I reached a contradiction. So, that meant R1C3 is actually 7. From there the puzzle unraveled. Now I am interested in seeing if that cell, R1C3 forms part of a logical pattern Simon can use to solve the puzzle, "logically." No, not really. Simon uncovered a beautiful double-finned x-wing of 5's. The resolution of that x-wing exposes the importance of the 7,8 in R1C3, but does not resolve it. It's resolved further downstream, very much at the end of the solution. The puzzle, as Simon notes, is very well crafted. By using Mark's approach, I was able to pin point it's weakness and cut to the chase. The R1C3 stood out, and yanking out spilled the goods without the clever disentanglement Simon went through. Seems unfair, but if you can find where the puzzle is vulnerable, a lot of the subtleties in its design can be lost. Mark's approach is like a bowling ball. As long as you don't shoot a gutter, you can do a lot of damage even if you don't throw a strike.

[@charleshultquist9233]

that's a little too much trial and error IMHO

[@annae7071]

Could you tell us where to find this sudoku software? So cool！

[@ewetho]

Now what gets me in this whole mess is how did the left side of the X-Wing resolve with neither space being a 5? Row 7 ended up as the column 5 - 5 not row 3 or 8?

[@ewetho]

Ahh the Fin!

[@wpbn5613]

What's the difference between a naked single/pair/triplet and a hidden one?

[@glennmelven3414]

Naked single is when only one number can go into a cell. You are able to see that you can't put any of the other 8 digits in that cell. With a hidden single you will have 2 of that same digit pointing to a cell and it can only go there. At about 2:35 he places what he calls a hidden single 4 in R1 C2, but that is a naked single. At the very beginning he puts a hidden single 6 in R2 C2.

[@azizulhakim5248]

I dont understand, if r3c5 is 5, that doesnt make r8c8 5 isn’t it? That means r1c8 could still be 5 16:54

[@azizulhakim5248]

And also, i have a few sudoku puzzles that i got stuck. How can I ask you for help? For your next vid maybe

[@MissKapanadze]

If r3c5 is a 5, then the only place in row 8 that a 5 can go in is column 8

[@desyncer]

make sure to remember the 2,5 pair in c6. that limits the places you get fit a valid 5 into r8.

[@lawrencekallal6640]

Nice puzzle, thanks for posting that. Always enjoy your challenging puzzles and exploration of the various techniques for solving them. In your September 20, 2018 video titled _A New Sudoku Technique Unveiled: The "Schrodinger Cell"_ you tested 2 numbers in a square to see what numbers would be forced into another area or box of the puzzle. kzbin.info/www/bejne/mJ-znqB3msdgias I like to call this method a force test, or force testing (FT), as that is basically what it amounts to. By testing the implications of selecting either one of two possible numbers in a square, or a twin pair in a 3x3 box, or even a number or twin that can only be placed in 2 positions for a row or column, and seeing if anything is resolved looking for -- a contradiction or conflict. -- some number that is punted into the same spot no matter which selection one makes with the FT. … one can sometimes very effectively resolve difficult puzzles. The trick of course if finding that magic FT that breaks the puzzle open. In this current puzzle there is a nice FT that breaks the puzzle open -- no XYZ or W wings needed. Starting at 5:40 with the 149 triple in the top middle block, and testing the 19 pair in RC24. If we try and put a 9 in that square, pair the 1 and 4, and resolve the implications in that block and the bottom middle block you end up with a 3 in RC92. The 3 pairs needs to be placed in the bottom right block and middle block to do this. This 3 in RC92 forces the 7 in the top left block into row 1, but the top middle block also will have a 7 in it -- leaving a contradiction or conflict. So, RC24 has to be a 1. Placing the 1 in that square resolves the 1,4, and 9 in the top middle block and effectively breaks the puzzle open.

[@lawrencekallal6640]

Actually, you don't even need to place those 3 pairs as once the 1,2 and 8 are placed in the bottom middle block with the FT, there is only 1 open square left in row 9, RC92 into which the 3 goes.

[@biharilalgupta2905]

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