Is This Killer Sudoku Really Impossible?

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Hi! We're Simon Anthony and Mark Goodliffe, two of the UK's most enthusiastic puzzle solvers. We have both represented the UK at the World Sudoku Championships and the World Puzzle Championships. We're also "cryptic crossword" aficionados. Mark is the eleven-time winner of The Times championship and Simon is the former record holder for most consecutive correct solutions to The Listener crossword. We hope we can help your puzzle solving while also introducing you to some of the world's best puzzles.

Thank you for watching!
Simon and Mark

[billiardsprofessor]

Thank you for sharing the puzzle! For clarity, the puzzle was included in an iPhone app called SudokuKiller, and "Impossible" is their most difficult category. I got to about the same as you at around 11:37. Just before you "spotted a nice trick." That was very elegant!

I am so glad I found your channel. I very much enjoy sudoku, as well as cryptic crosswords (although I haven't done one in a while), and your videos have actually helped me solve harder sudokus. I especially like your techniques of pencil marking corners vs centers.


As for my moniker: I am a pool player and an instructor, and one of my friends calls me "professor" all the time. So, of course, I decided my pool instruction business should be called Billiards Professor :) I truly love puzzles and games that make you think.



Thanks again!

[TheArizztokrat]

I find it very nice that you always show your train of thought so we can comprehend and learn.

[Pianoblook]

This has quickly become my favorite channel to watch daily. Very wholesome and satisfying :)

[gposchman]

I still do not have all the number sequences memorized for numbers that fall with a single group size. It took me quite a while, and I only missed how the 9 in the 45 group affected the middle box. Working out the possibles for the other two larger 7 groups took a bit of time. I would pause my game and watch the video until you caught with me, of course, you were paused while I played. It took me 2 hours. But when I figured out the 5&8 in the lower middle box, it all fell into place. A really great puzzle and I am ever grateful that I support you on Patreon, well worth the $3 and I love your Sandwich Sudoku game. I recommend both to all who play Sudoku.

[tone]

Didn't catch those chaining 2s so I got stuck for a long time trying to work out the bottom box, eventually realized that the 239 in r8 meant that one of the numbers in the 33 cage was _either_ 3 or 9, which meant that wasn't the 12 total that was missing, and coupled with some other reasoning I'd already done that forced a 9 somewhere into the 23 cage, that was enough to collapse it. Took ages, to get there, though. Great puzzle!

[Pyromonkey]

That was over an hour of struggle for me. You found the 2-7 in the 9 cage so much faster than I did, and frankly, I gave up and bifurcated on the 5 in box 5, which I got lucky with on it being correct the first go around. It solved the puzzle in the end, but I was unable to spot the logic that you did. Perhaps with more killer sudoku experience I'll be able to in the end!

[marcohofmann]

I rarely comment on any YT Vids but this was just a beautiful solve! You can see Simon enjoying this so much at 21:04. I love seeing this. Great job!

[BlueCyann]

1:39:08 I don't think I could have completed it at all a few weeks ago when the video first went up, and even a week ago think I would have resorted to filling in a lot more little numbers than I finally did, trying to figure out that 27, or the location of the 4 in column 7 (which was my replacement for what the video did with 5s around 16 minutes).


20 minutes blows my mind. I'm glad I'm finally learning what to look for. It just takes so, so much longer. (Some of it is backtracking mistakes, but not that much. At least this time I could find them.)


Oh also: the challenge about the 18 pair: there is in fact also a problem with the 1 and 8. Putting a 5 in the 23 cage forces r9c7 to be an 8. Having an 8 there means the only spot to put an 8 in the 33 cage (which needs an 8) is r6c6. Which the 18 pair is pointing right at.

[oNtuobAwoH]

I was completely and utterly stuck until your deduction in the lower right box and the 33 cage. Would have never, and I mean never, realized all of that. Awesome video.

[willconyers]

That was some quite impressive logic at 17:30, would have taken me forever to spot that myself

[darkguardian]

This was my first Killer Sudoku, came in just over 2 hours (give or take for brain breaks). Enjoyed watching your solve after to see how we differed on approach.

[chipsounder]

With Simon, nothing is impossible, you are definitely one of the best channels 👏👏👏

[RandomBurfness]

Lovely puzzle! Thanks for sharing this gem. I especially liked the 33-cage in in the lower-right part of the grid.

[CzerniawskiMateusz]

13:45 after puting the 1 there was little reason to go this far with the kinda crazy logics because you could have put 1 in the upper central box with normal sedoku.

[thetacog]

Took me 20 minutes to place 3 digits. Your logic is just incredible. Then again, this was my second ever Killer Sudoku. The first I tried took you 40 minutes so maybe I should just try easier puzzles...

[msliz]

Went straight to your website and solved it in 53 minutes 8 seconds. Felt real good until I realized you solved it in 22 minutes

[honigschlecker]

He needs minutes to spot the obvious (4-7 in box 8). And then he comes up with two mindblowing deducations within seconds. Especially the second one starting around 15:50. How can you spot such things?!

[XoIoRouge]

I boiled so many of this down to various pairs. Just so many pairs that couldn't unwind. And the 5-8 pair at the bottom row 13 cage would control everything. I spent 2 hours on the webpage which timed me (2hr18m) but (1) I didn't know combinations by heart so I had to google search many of them and (2) I wasn't really working on it all 2hr and 18min, as I took a few breaks. I suspect it actually took me somewhere between 1h30m to 2hr to solve.

I never got the center row as you did, and I ultimately solved it by guessing 8, 5 on the 13 cage and then another guess later. The 8, 5 guess was correct, but my later 3, 6 guess in the middle column was reversed, -- led to an impossible row 4; swapped the two values I guessed with and solved the problem from there.


Now to watch the video and actually learn.

[bfishryuhayabusa]

I got the 1-5 pair in the left middle block by adding the complete cages in the first two columns to get 84, and since the 21 cage had a 4, the only possibility was 5 and 1.

Once you get the 4, 2, and 9 in the 45 cage and also that key 5-8 pair, the 5 cells of the cage that are in the center block have to contain 1, 3, 6, and 7. More importantly, this means those numbers cannot be in the right three cells of that block. The 1-6 pair in the bottom center block then restricts the 1 and 6 of the center block to the middle column. This resolves the 11 and 9 cages in the top middle block, and the resulting 5 means that the only remaining place in the center row that can take a 5 is the 11 cage, which is therefore a 5-6 pair.

I know I got the 2 and 7 in that middle-right 9 cage somehow, but I can't figure out what I did, though I quite like how you managed that.

The real killer here is resolving the 5 in the 35 cage. It's the only number that has to be in that cage from the start, and I narrow it down to the two cells, but that resolution is quite hard to see. I bifurcated at that point, as much as I hate doing that.

[andrewtaylor]

I'm kinda surprised. When I was going at the puzzle, I ended up using the 7th column with the 33 and 35 cages a ton to finish up. Once you find 9, 7 and 5, everything that remains in the column is in one of the two cages. For example, since the bottom right preoccupies a 4 outside the column, it forces a 4 (and thus a 6) into the 35 cage. However, there's only one spot for a 6.