Diabolical sudoku: Finned X-Wings

Simon takes a look at The Daily Telegraph's "Diabolical" Sudoku from 27 July 2018. There is a beautiful piece of logic to finish off the puzzle using a finned x-wing.

[@cornycornsnake]

The words that appeared on screen in response to what Simon said at 8:01 made me laugh out loud.

[@goldenera7090]

@6:12 when Simon deduces that 9 can't go in rows 8 & 9 column 2, we can also deduce that based on row 5 column 2 cannot have 9 this means row 2, column 2 will be 9 and row 3 column 7 will be 9 and row 8, column 8 and row 9 column 6 will be 9

[@ThatGuy-dj3qr]

That struck me too. It is always easier for us without the pressure of time, and viewing the puzzle from a different perspective.

[@bhupendersingh6604]

He doesn't deduce 9 can't go in r8c2 and r9c2.

[@samcappleman-lynes6914]

Hi Simon! I love these Diabolical showcases! I have a few suggestions about this puzzle based on my run-through of it. - The detour near the start that led to the placement of a 4 can be seen as yet another (Sashimi) Finned X-Wing! (In columns 3 and 8) - At the point where you mention the uniqueness constraint, you can immediately place a 9 by looking at locations in column 2 (as opposed to the block, which you focused on) - There's an alternative killing move available which I think is easier to spot. Cells r8c2, r5c2, r6c3 and r6c5 all hold 6/7 pairs, which in effect makes the two ends (r8c2 and r6c5) a remote 6/7 pair. That places a 5 into r8c5 and finishes the puzzle!

[@CrackingTheCryptic]

Sam, thank you! Your alternative killing move is absolutely lovely. I am definitely going to do a follow-up video just explaining this point - fantastic stuff!

[@JohnRandomness105]

Unfortunately, I didn't notice either finned X-wing. I followed Simon's chain placing the 4. With the buttload of 67 pairs near the end, I think that I substantially followed your alternative method. I colored the 67 pairs yellow and blue, and saw a yellow cell facing one of a yellow-5 domino. That placed the 5.

[@shadower3833]

Thank you Simon for your explanation, but I lost you from 11:36 and on. Now look at this solution: There are several boxes with only 6 and 7, especially r6c5 (you left it empty, but contains only a 6 or 7). Now this one, together with the alternating box r8c2 (if one is 6, the other one is 7, and vice versa) makes it impossible for the box they both see, r8c5, to contain both 6 and 7, and therefore r8c5 must be a 5, and r4c6 must be a 5, resulting in the 1,8 pairs on row 5.

[@stevefrench8512]

Great channel. At 12.20 you highlight 2 green squares and say a 7 must be in one of them, but surely a 7 can't be in the top green square because of the 7 in row 4 Column 7. Just starting to learn the advance sudoku guides.

[@gemmadidit4118]

I just learned that projection (what if?)is a thing. I considered it too much work...but I guess it's part of the process. Sudoku is a completely MINDFUL process. You have to surrender to the work and let go of expectation to really enjoy it. My app gives clues but I hate to use them and I don't want to guess. The greatest satisfaction is when notations reach CRITICAL MASS and everything starts coming together...and suddenly I'm in a state of FLOW. I often treat problems in my life like a SUDOKU puzzle. It very often WORKs for me!

[@Prometheus720]

If you do the virtualization for the 1-4 pair, you find that in either case a 4 MUST go in the top middle cell in the top right block. That blows my mind. I'm not a very good solver but I have never thought of virtualizing a pair to find a convergence.

[@michielgerretzen9777]

Instead of spotting the finned X-wings on 6's and 7's, a breakthrough could be reached by spotting a finned X-wing on 4's in cells (3,3)-(3,7)-(5,3)-(5,7) and the finn in cell (4,7). Either there is a true X-wing or there is a 4 in (4,7), both leading to the conclusion that a 4 can't go into cell (5,8). Not as beautiful and effective as Simon's finned X-wings on 6's and 7's though.

[@ThatGuy-dj3qr]

What I cannot seem to get my brain around from about 10:40 onward in this video is that all logic is based on the assumption that there will either be a 6, 7 X-wing or a finned X-wing involving columns 2 and 5. However, it seems to me that there could also be a 6,7 X-wing involving columns 2 and 4 (instead of column 5). I cannot seem to be able to understand why Simon is able to assume the the X-wing could only be in column 5. Can anyone help me with the logic that says we won't test column 4 for the x_wing?

[@cornycornsnake]

Does the logic on 4s from the 4/1 pair leading to the placement of the 4 in row one starting at time 2:33 count as bifurcation? I’m reviewing past Vids, trying to understand what does and doesn’t count as bifurcation.

[@moshehim1000]

How on Earth did you know to look at the 4s (as opposed to any other number) and at those 4s specifically (as opposed to all other 4s)?

[@ChessRabbitt]

You had 7s on the bottom when you got your 6-7 pair in the bottom left square. You could have solved this problem long before you went in to your explanation about the x-wing.

[@goldenera7090]

@ 12:11 simon shows he is real genius

[@meenda7031]

9:27

[@javabeanz8549]

Oh cool I am the first to view!