Is This Killer Sudoku Really Impossible? 

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!

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

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

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!

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.

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!

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.

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

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.

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.

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!

I solved this one a while ago -- it took me almost 3 hours... :-) The funny thing about this app is that some of its "easy" puzzles are harder than its "very hard" puzzles...

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.

This took me 54:35 holy crap what a hard puzzle. I had to resort to the calculator on dailykillersudokus.

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.

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

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.

When I saw that 33 cage I wondered how you could have a 2 in that cage. To many Sandwiches I guess.

I would never have eliminated the 5 on the right of row 7. Well done, Sir.

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...

done ... i mean opening the puzzle.

I found it rather easy. I still took around 3 times your pace as usual. I played the 35 and 33 cage off of each other with the focus on the 35 cage more, eliminating 2 possibilities forcing the 35 cage contents for 4 of the 7 cells and likewise for the 33 cage. That play quickly resolved the 58 combos for the 13 cage at the bottom and the upper left cell of the middle bottom 3x3. Unfortunately this method took a bit of mental tracking (and for the first elimination I had to mark a few and then undo those marks) so it was a bit slower than your elegant solution. Like you said, a rather nice puzzle.

I tried it and got stuck also. It was your trick identifying where the five went in the lower right box that broke the puzzle open. Wish I could spot those things like you can!

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.

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

Couple of things I deduced differently: In row 5, 5 can't go to the middle because the total would be too big. It can't go in the 9-cage because 4 is used up. Therefore, it has to be in the 11-cage (and the other digit of course has to be 6). Another one was what would happen if I put a 5 in r6c6. If the 33-cage contained a 5, it would have to contain a 7 as well, but there is no place to put it.

New to killer sudokus, had to laugh at how you breezed through this 5 times faster than me.

I didn’t see the 249 triple. I instead summed the 4 right columns and found 4 of the cells subtracted to 17, meaning their corresponding pairs subtracted to 19. Having pencil marked a 2 in them already, it greatly reduced the possibilities in the remaining 3 cells, and fed back into the aforementioned pairs. I missed the later trick, instead reasoning that if r8c9 had been a 3, it places a 9 in r8c9. Clearly a problem, so the 33 cage couldn’t leave out a 3 and 9. Also that cages 1 positions fixed the 18 pair in the 9 cage above it. Took me over an hour.

i didn't get the 2 trick in the middle of the video but used a whole different logic. with a couple kids break it took me almost 2.5 hrs (minus 45 min), still cracked my head a bit on it. i paired the board until it croumbulled

The puzzle is one of the free ones on the Android app called Sudoku Killer.

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

This puzzle was really excellent! I proceeded differently for the bottom middle block (much more complex reasoning, lost lots of time there) and for the right side (quite simpler reasoning, on the other hand). I find you go a bit too slowly pn some very obvious stuff and a bit too fast on complex reasonings (had to relisten at times for some parts of your right side solving).

Thanks for this video. I saved it as a "tutorial" to explain how to solve a sudoku killer puzzle.

A good puzzle and you were in form today so a very nice solve.

My first complete solve before starting the video :) Took a longer path though...

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?!

Phenomenal puzzle! I had to give up because I didn't find the trick involving 2's

That is so funny - it took me like 1 second to realise that this exact variant was the very first killer sudoku I came across) And the one which got me interested in solving puzzles in general. I solved it in about 2 hours, spending most time just staring at the grid and occasionally "inventing" new techniques and putting in another number. Knew nothing about sudoku back then.

Thank you !!! I was also stuck on this one

i am too lazy to calculate these types of sudoku, i like thermometer ones much more

That was cool, makes me know I can solve the one I'm on somehow

This was very pleasing to solve. I enjoyed it very much.

I need to memorise the combinations to spot this 😂

At 17:43 I understood all your logic to not put 5 into these 2 cells into the box but then how do you know the 5 isn't in r7c7 but in r9c7 ?? i don't see it

I'm late to the party but I do have a question...when I play sudoku I do so with the assumption that the numbers in a numbered cell CAN in fact be repeated (provided they're correctly not placed in the same square, line or column), and it is so in many sudokus I played even on paper. I was stuck on this one because of that, and I was surprised by the fact that it seems normal to you to assume that none of the numbers repeat.

Got stuck at the part with the logic about the 5 in the bottom right. First I also thought about putting the 5-7 pair in the 23 cage but didn't see the logic with the 1-8 pair. Def a good but difficult puzzle

Why are 2 and 7 at 13:10, I think it may be 8 and 1, please advise.😊

At 11:45 you noticed that there must be a 4 in either of the r9c8 and r9c9. You then drew two conclusions that I can't follow along with (1) that there must also be an 8 in the cage (2) that there also needs to be either 3 and 9 or 5 and 7 in the cage. Any help would be appreciated!

The 5 on 14:15 is a mean mean mean trick... I had to see it for myself... I've summed all 36 of 7 out of 9 combinations, and only 3 of them sum up to 33 in 7 digits, two of them both have 7 & 5, the third one doesn't. You either know this, our you're bust :S

i need to work on how to make numbers, and then working out how to fit them in the grid the correct way around

Wow, took me about an hour but I managed surprisingly.

Didn't find that elimination of 5s in the bottom right box. Got stuck there.

it is funny to see how you mind works!! you come up with such high tier thought processes but you often completely miss the obvious resolutions.. this 11 cage in the center bottom you had it as soon as you put the 2 and 9 on the central square, meaning so much less to think about that you actually did.. same goes for the 5 cage and you let those 4 and 7 pending for almost 10 min not resolving the 4-8 in the 12 cage on the left bottom square... that is always surprising as seeing those obvious resolutions would ease your thought process so much! I think you jump into new calculus too fast and lack taking the time to actually look at all the solving you get from putting a new number, making you life so much harder ahaha!

Now I know what to get you guys for Christmas :-P

At 7:20 when you say you couldn't figure out how to resolve it, can't you use the 11 cage in the bottom middle 3x3 box to rule out a 2, as it would place a 9 in r8c6, which conflicts with the 9 pair in r6?

I suggest looking at the other boxes once you’ve resolved one. Throughout a lot of puzzles I’ve watched you complete, I have noticed you frequently don’t check other squares once you figure where a number goes.

I dont know if i just happened to stumble upon and see things on accident and got lucky. But i feel the puzzle wasn't that impossible. Anyways, love the videos as per usual, keep it up

Took me over an hour to get this one

11 hours 55 minutes 18 seconds. Not a good time, but I did it, lol. Would have been good to know that numbers could not be used twice in a cage before I started........damn

@17:02, how did you rule out it cannot be 5?

Longest one yet for me! Hour and 3 minutes :D

It appears that I completed this in the original iPad app in 92 minutes. I have no idea how much of that time was spent with the puzzle open while I was distracted.

4:52:28 with some breaks

Little bit over 3 hours but Ive made it!!! Hurray!!! :-D

Could someone please explain the logic at 17:20 to me, i cant see how the 9 is ruled out from the rightmost 239 pair

i did this like 4 days ago, took me 1:30, so tricky

i really don’t get how you ruled out the 5 to not be in the 33cage when placing that 7 in col7

Took me an hour and five minutes, but I managed to solve it. :D

Resolved 20.47

Is "killer" giving you problems with the youtube algorithm?

1:28:44

This one is far from impossible. Even I finished it before.

2:33 winner :-) Sadly not minutes but still, I got there.

Ouch, I paused it at 1st second and went to solve it. Took me 3,5hrs give or take, but I managed. Then I checked the video and I found out about the rule that there are only unique numbers in the regions >.< Solved again with the new knowledge, cut me down to little over 1hr. Nowhere near your time, unfortunately.

You could give me a year n I wouldnt be able to finish that

No it is not. It took 2 tries and something like 3 hours on and off, but it is possible.

No, is not impossible. I took an hour and about 8 minutes, but it can definitely be solved by a human solver, not just a computer one. I ended up resorting to calculating how a few squares could add up to 30 given the numbers I could use.

First!