Lol me. So I was correct in putting in the disclaimer. You are right, all that this shows is that r1c3 can't be a 2, which shows the 5 is correct, but not for the reason they are trying to demonstrate.

Interestingly, the puzzle can be used to demonstrate another thing. R1C3, R1C6, R4C6, and R4C3 form a square that uses the same four digits in pairs. It works a bit like an X-wing: there can be no other 2s in R1, no other 7s in C6, no other 4s in R4, no other 1s in C3. This leads to some eliminations.

Yeah, I don't think I would have picked up on that. I am not too swift on the "same principle but slightly different" type of Sudoku deductions/eliminations. Lots of times I initially miss seeing some of text book solving scenarios (I.e Swordfish and XY).

Thx for looking at this Eric.

Clearly, nobody loves beating a dead horse more than I do.

There are simpler ways to eliminate the 7 from the cell above the target cell in the example (empty rectangle technique). But the best thing is that using the square I pointed out causes the whole puzzle to just collapse.

Eric

A question for you if you have time

I have been getting into classic Sudoku again and did this one recently. I solved it then viewed how Mark solved it.

It seems he took a rather odd approach and one might argue he intentionally avoided a path to a solution that might have shown the puzzle had a flawed design.

If you go to about 4:40 on the video you with note he has a 59 pair in a row but decides not to mark the 59 pair in column 9. He even says he is not going to mark it. That is really odd because he harps on the fact always that when you have just two candidates in any cell you mark them.

At about 7:20 he notices a 59 in the same column that has the original 59 he found. It would seem to me the next step would be to treat the three 59s like an XY wing and then explore what the cell in C9R1 could contain. That is what I did at this point and noted it was a 459.

So I had on 4 corners 59/59/59/459 and if I am assuming the puzzle has only one valid solution, then C9:R1 has to be a 4.

Is this a fair way to solve a cell? IMO yes, because you can't unsee or unknow what u see.

But, the question for you is do you think Mark saw this and just avoided looking at it so he would not expose it??

Thx

https://www.youtube.com/watch?v=DZXg_hTLhX0

My take on this is that Mark is, at heart, a speed solver. As can be seen at the beginning, his approach is not systematic, rather he takes a broad look at the board for any obvious wins. I think that he doesn't mark the 59 in box 6 because it doesn't do anything for him in the box, so he just moves on and looks elsewhere. I think he marked the 59 pair in box 4 because it filled the box. Four years later on, I feel he would be marking all pairs more rigorously.

As to trying to use uniqueness, like you did, I think it didn't occur to him here. I'm not sure at what point in their history they decided to avoid uniqueness based deductions, but it was very possibly before this video.

I don't think Mark directed his solve to avoid anything, it was just the way it unfolded. On another day, he might have done things differently.

As to whether uniqueness is a fair way to solve anything, I say that is up to the solver. Personally, I do not find it a satisfying way to solve a puzzle, but it is in no sense improper.

Thx for your time Eric

I agree with your comments about the process in general. I have watched more than a few solves that have gone..." wait, I was just thinking where the 6s might go, and noticed an X wing..." . This, at a time when many solvers would still be half way through the first go filling the pair candidates.

As for the uniqueness thing in bold, I agree. Well said.

I'd rather not have seen it, but it is pretty much impossible to ignore it once you have. And, yeah - there is no satisfaction in finding and using it.

The NYTimes Hard Sept 9th puzzle was a good one today..

I've learned that one of my puzzles will appear on YouTube, Tuesday at 1pm central time. The channel is "timotab". I believe the puzzle appeared several months ago on BremSter's puzzle channel also.

Sweet!

I just found out today that there are three Youtubers that have posted videos of my Airport Fog puzzle.

I submitted the puzzle (although it was an earlier version by mistake) to BremSter.
https://www.youtube.com/watch?v=5hH0FwIBaXU

About a week later, I discovered that Clay Loves Logic made a video (unknown to me).
https://www.youtube.com/watch?v=sPsfW5oYxXE

Today I saw a reference to Puzzle Patzer's video (also unknown to me).
https://www.youtube.com/watch?v=8kT-zY6w8Co

I think it is because fog puzzles are fun, and not usually too difficult (especially mine). I do wish they would try to contact me first, but they probably can't find me easily.