Is there an Optical Illusion here? You Decide

[@benjaminvazquez1243]

One of the things I love about CtC is that Simon almost always completes the colouring, while Mark almost always removes it when it's done its job. I feel it exemplifies the difference between the way the two men approach the puzzles. Mark thrives is sudoku. A good clean grid lets the numbers shine through. Simon, on the other hand is always looking for the weird twist to the puzzle. He treats even the most sudoku-y puzzle by focusing on the logic inherent in the variant. Completing the colouring highlights the non-sudoku structure in the puzzle. And I feel you can see it when they make exceptions. Occasionally, when the pure sudoku elements do dominate a puzzle, Simon will remove the colouring. And occasionally, when there's a really magnificent twist to a puzzle, Mark will complete the colouring to highlight it. Like a nod to the nature of the puzzle itself. It reminds me of the different strengths they bring to the puzzles every time.

[@emilywilliams3237]

Such an interesting set of observations on the two of them.

[@tBagley43]

simon seems to be more of a big picture oriented solver, while mark seems to be more detail oriented, and that definitely aligns with their marking habits

[@dkamm65]

I think the key to the puzzle is a byproduct of the extended Phistomephel ring. The digit in the R5C5 can not appear in the Phisto ring by sudoku/knight constraint/diagonal, and by extension can not appear in any of the corner 2x2s because the 16 digits in the four 2x2s are the exact set in the Phisto ring. This would have removed 5 from R5C5 very early on as 5 is neither odd nor can it go on an X. This would have also forced the Xs in box 9 and box 3 to be 4/6.

[@brianmcadam443]

To put on my "theory wonk" hat for a minute, this is a stronger variant of Potatohead's Theorem. The original was "in a puzzle with *either* a knight's move or diagonal constraint, any digit that is not in Phistomephel's Ring *must* appear in the central cell, and there can be at most one such digit". But it is still possible for all nine digits to appear in Phistomephel, and thus no special significance is assigned to the central cell. Kedad's extension of that here is that in a puzzle which has *both* a knight's move *and* diagonal, the ambiguity is removed. The digit in the central cell *cannot* appear in the ring, and therefore must not appear in the corner boxes.

[@uigrad]

I think it's pretty easy to skip it also. One of the early keys is seeing where 5 can go in column 7, and how that affects the middle box. Mark used the Potatohead theorem for this, but it's easy to see without it. The next big key is coloring 4s and 6s. The one that is not in the center of the puzzle has to go on the Phistomephel ring, but even without knowing about the ring, you can simply look at row 3, and find that one of them has to go on the x (columns 8 & 9). Altogether, it's a very beautiful puzzle. There are multiple different options for getting through the tough parts, and each of them are beautiful in their own right.

[@inspiringsand123]

Rules: 02:10 Let's Get Cracking: 03:48 What about this video's Top Tier Simarkisms?! Phistomefel: 3x (27:53, 28:03, 28:14) Three In the Corner: 1x (51:26) And how about this video's Simarkisms?! Ah: 14x (06:01, 07:18, 09:23, 18:00, 20:09, 22:10, 23:01, 23:01, 24:42, 28:47, 33:25, 48:31, 49:38, 50:32) Bother: 7x (24:47, 31:29, 31:29, 31:29, 31:29, 31:29, 38:09) Sorry: 6x (06:56, 19:19, 23:01, 23:41, 33:33, 47:45) In Fact: 6x (11:02, 14:52, 23:52, 31:41, 42:07, 47:22) Clever: 4x (10:09, 11:22, 35:49, 52:22) Brilliant: 4x (01:41, 12:51, 37:06, 37:06) Obviously: 4x (03:25, 05:22, 05:58, 33:48) Weird: 4x (12:02, 14:09, 23:58, 46:15) First Digit: 2x (10:06) Pencil Mark/mark: 2x (23:09, 45:03) Goodness: 1x (47:16) The Answer is: 1x (32:11) Stuck: 1x (31:35) Ridiculous: 1x (43:25) By Sudoku: 1x (45:23) Intriguing: 1x (52:31) Progress: 1x (31:33) Wow: 1x (28:59) Fabulous: 1x (01:11) Symmetry: 1x (10:10) Most popular number(>9), digit and colour this video: Twenty Eight (15 mentions) One (74 mentions) Purple (27 mentions) Antithesis Battles: Low (11) - High (6) Even (12) - Odd (10) Black (9) - White (5) Row (23) - Column (22) FAQ: Q1: You missed something! A1: That could very well be the case! Human speech can be hard to understand for computers like me! Point out the ones that I missed and maybe I'll learn! Q2: Can you do this for another channel? A2: I've been thinking about that and wrote some code to make that possible. Let me know which channel you think would be a good fit!

[@chocolateboy300]

I finished in 33:03 minutes. This was a really nice puzzle that flowed very well. I think my favorite part was the break-in and noticing that 5 could not go in the center cell, because 5 is forced into r7c2, which bounces around the grid before being forced onto the X clue in the top right. The 46's fell pretty fast next. My next favorite part was using the 12's and ruling out a 2 from r5c4. I had already ruled out a 2 from r6c1 due to the 2 on the negative line being in only two spots. This forced the 2 in r5c4 to be in r4c3. The 2's bounce around the grid and force two 2's into columns 89 of box 3 and 9. This leaves nowhere for 2 to go in box 6, thus breaking it. That was so cool to see. This was a very good puzzle for a first time setter. I enjoyed it very much. As always, it feels good to beat Mark's time. Great Puzzle!

[@therealnaz6611]

This makes absolutely no sense.

[@markp7262]

37:21 finish. A very nice puzzle. I used the ring when I noticed where 5 had to go in box nine, and then to run the center cell around the corner boxes, but that was it. Excellent job!

[@Mujaki]

29:05 Red Five, Standing By!

[@compiling]

Solved in 57:44. A neat bit of logic early on is that the central digit sees the entire Phistomefel Ring, so that digit needs to be kept out of the corners of the grid. That let me sort out the X clues very quickly, but the rest of the puzzle was really difficult.

[@anaayoung9142]

32:12 amazing first puzzle!! 😊

[@emilywilliams3237]

I think that some of the harder puzzles with knight's constraint do devolve into searching for the one cell that sees a key digit and therefore moves the solve along, sometimes only incrementally. Not at all easy! It's a pleasure, always, to watch you solve one of these tricky ones, Mark, and I think that this sort in particular needs pencil-marking to bring relationships to light. Thanks, as always, for the video!

[@brianmcadam443]

This always makes me a bit wary of knight's move puzzles, especially from novice setters. Because I have seen *many* examples where a knight's move is applied to force a unique solution on a puzzle that feels "incomplete" in some fundamental way, where it would have been far more satisfying to apply more of whatever other constraints were used for the break-in. And when the "endgame" turns into hunt-and-peck single candidates it's a grind. This puzzle, IMO, neatly avoided the trap. My solve path was very different from Mark's, and the knight's move figured much more prominently in the early game, which made it feel integral rather than stapled on. And there were enough other clues in the grid that I was able to finish smoothly, without the grind.

[@sjotpit3521]

You could say purple was in the x in box 3 which made it a 4 6 pair and that would probably make the solve quite a bit easier

[@Hakucho64]

Needed Mark's help a couple of times but otherwise muddled through. Nice puzzle, although as Mark observed, hard to know what to look at at any given moment.

[@Coyotek4]

50:33 ... first created sudoku, eh? Nice puzzle!

[@PinkSupervisor]

Done in 39:10. This is probably the first time I finished ahead of Mark. I managed to place all 5s and the possible spots for 4 or 6 really quickly (more colour = more good). Took a bit to figure out where to look next but by the end I was too quick to catch up with my colouring.

[@titusadduxas]

53:56 - Happy with that.

[@piarittersporn]

Beautiful puzzle.

[@bristolrovers27]

Excellent puzzle - brain wracking but very well constructed

[@Kirbyfan87827]

Finished in 49:26. This was a lot more fun than Simon's puzzle!

[@Gonzalo_Garcia_]

15:50 for me. Nice break-in!

[@adrianhead6272]

The knight's move is always so powerful... making the puzzle far simpler. Completed in 17m47s.

[@Mephistahpheles]

Spoilerish: I think removing the colour from the German whispers was slightly premature. Much can be done by looking at what the bits sticking out of box 5 see.

[@wmchristie]

I’ve never implemented the idea that speaking my thought process out loud, as Mark and Simon do, would aid my solves. I do realize that neither of them need to speak in order to solve, but I am a mere human. I’m commenting as much to put some form of action to my thoughts as anything. My girlfriend will think I’m even more weird when I start narrating my sudoku. 😂

[@wmchristie]

As a software developer, talking to the duck has frequently been reliable en.wikipedia.org/wiki/Rubber_duck_debugging

[@bait6652]

Wow gut wrencher for me after working all the nonknights rules

[@tBagley43]

33:17 great for a first puzzle :)

[@oduwatcher]

I first proved the middle square couldn't be a 5, then chased the 46 pairs around the grid. I think that was an easier way to do it

[@chipsounder4633]

Wow, watching along and completely stuck 😂 then as soon as you mentioned phistomefel and potato head's theorem, i looked at the 5 in row 7 and acclaimed 'thats got to be it!' 😂 excellent puzzle.

[@Thegeektoendallgeeks]

01:41:03

[@donatello_8691]

I got to simon this puzzle, overlooked the easier deductions, and was able to deduce 5 could not be the center digit, or their would be no 5s placeable in row 3. Now watching mark i wonder how i missed some of the logic

[@srwapo]

When you mentioned phistomefel and started coloring what couldn't be purple due to knight's move, I saw that it wasn't on the ring and therefore couldn't be in the "corners" of boxes 1,3,7,9, and then had to on the X in row 3.

[@ichselbst4798]

I could complete the 5s due to knight move and after deducing the 3 in box 8 I also could complete them totally. So it took me less than 17 minutes

[@kathyjohnson2043]

that is an impressive time; are you usually that fast or was this the exception?

[@ichselbst4798]

@@kathyjohnson2043 In Knight Move puzzles and Fogs I am pretty good in general. Somehow the colouring methode fits my way to think. If i was about to rate myself I´d concider a draw since there are alot I need more time.

[@adrianhead6272]

@@kathyjohnson2043 It really wasn't as difficult as Mark made it. The knight's move is so powerful.

[@wanderlustwarrior]

38:03 for me on this one.

[@OlafDoschke]

Diagonal sudokus always looks a bit like the Star Wars trench run, even more so with the two vertical lines. So, spoiler: Use the 4s.

[@tms1624]

Mark this day as one of the few I beat Mark's time: 37min

[@kennetsdad]

I fully approve of this puzzle. 👍

[@JohnBodoni]

you're not imagining the optical illusion.

[@stupidas9466]

ALL optical illusions ARE imagined, by definition.😊

[@bibliopolist]

Somehow I got all the fives very early, but then continued to fail, and had to look at the video to find out that the thing in box 3 is not a thermometer... my fault of not really reading the rules...

[@kathyjohnson2043]

Yes, that is a known optical illusion.

[@EiQ200]

I stopped after cracking the brain after round 30 minutes. Then I remembered the knight-move. 🙄

[@TurquoizeGoldscraper]

45:58 for me.

[@Petrus74-yj4kv]

At 27:25, had Mark actually followed the trail of 8s starting in R2C5, he would've immediately seen it's wrong and placed 8 in box 2... but of course he didn't. Half-assed positions check is all he bothers to do. EDIT: Same thing at 30:10... How did he not notice that the purple 46 had to be on the X in box 3? So weird...

[@chipsounder4633]

Is that my long lost twin brother in row 7?

[@Timlagor]

Circle + Whisper != Thermo!