you said 1356 on row 8 column 2 that would show a 5 could only be there

@@rickwoods393 yes. a hidden single. 5 is only number can go in

It seems to be easier to pick up on their misses than it is to see our own when we;re doing it.

I know this is old, but i'm wondering the same thing!

I think he misspeaks - a 6 could also go in r5c1. (He then uses this as part of the finned x-wing.)

Have a look at the trick I go through starting around 18m45s into the video. If it still isn't clear, let me know.

Yeah, they use their own web app these days which has the centre notation (and just looks a bit nicer too). However, they used to only use the one in this video, I think it's called Duncan's Sudoku Solver?

It will nullify the puzzle, because you it will not have an unique solution. You would not be able to know which number goes in which cell. Sudokus can not have to possible solutions, so you can discard the possibility of that pattern.

I understand the uniqueness and that this cannot work. What I don't understand is how you know to eliminate the 1 and not the 5 and from Row 6 and not Row 3.I am sure it is obvious, but I don't get it. Thanks.

Sheesh, it waas so obvious I was over thinking it and , please ignore the dullard

He knows that the 5's goes into r5c79, so i 1 will feet in these also there will be 2 sulotions, therefore he can eliminate 1

The issue with having four 1/5 pairs arranged like that is that at the end of the puzzle, even after filling in each and every other square, you would be left with those four squares being marked as 1 or 5 and having no way of knowing which was which. You would be able to mark them as 1 or 5 and it would still be valid. "Uniqueness" means that in almost all hand-crafted puzzles there is only ONE correct solution, and this situation where those four squares could be a 1 or a 5 as you see fit and therefore having TWO solutions will not be valid. This allows Simon to eliminate the bottom pair from having 1's.

@@DougalAAllan, thank you so much for your time. I am working on it and realized what uniqueness is, a new concept for me. But in my mind the trouble still remains: the number 1 could be placed, in the bottom row, in the middle column, for instance. Unless...Oh, think I´ve got it! He says that those TWO cells can´t have the 1. Only those two, not the line. Thanks again, pal. I guess I have much to learn.

It's because the "empty" part is saying that there is NO 2 in the rectangle - not that there is no number at all in the rectangle. It is the lack of a 2 option in any of the squares in the 2x2 "rectangle" that means we can do the logic shown at around 21 mins into the video.

Uniqueness problem - the final solution cannot have (a) 5 in r3c7 and r6c9 AND 1 in r3c9 and r6c7; NOR (b) 1 in r3c7 and r6c9 AND 5 in r3c9 and r6c7. If 1 occupies either r6c7 or r6c9, it will cause one of the situations (a) or (b), and the puzzle would be invalid (i.e. solution is not unique).

It looks like an X-wing to me since I don't know the final solution at this point, so what information already known rules out both of the X-wing possibilities?

@@robertjohnson3011 Yes, it is X-wing for 5 but not for 1 (because r6c7 can be 1, 5 or 6.). But this is NOT the chain of thought to get the solution here. Please think of it this way: If the final solution has 5 in r3c7 and r6c9 AND 1 in r3c9 and r6c7 (case a), do you agree that by exchanging the two 5’s and the two 1’s in this final solution (i.e. you get case b), you will get another solution which will still satisfy the rules of Sudoku? If you understand this point, you will understand the meaning of “uniqueness problem”.

I see it now. r6c7 and r6c9 are the only available places for a 5 in that 3x3 block. Thus, if one of them were a 1, the other would have to be the 5. Same if either is a 2, but IF they were 1 and 5 in some order, then r3c7 and r3c9 would be 1 and 5 in the opposite order and any other square on the board that could see one of those 5s could also see one of those 1s regardless of which 2 were the 5s. Thus nothing else on the board would be able to constrain those two 1s from trading places with those two 5s and the puzzle would then have 2 possible "solutions", which is not allowed.Now, that's sophisticated logic!

@@robertjohnson3011 but i do find an application a long time ago that confuse me by giving sudoku that had multiple answer, so i thought uniqueness is not really rule.

Yeah I used to do that as well, bruteforcing it. I guess thats always kinda easy, but I think your not improving really using this, keeping yourself at a limited speed and understanding.

@@antono.8896, I don't care about speed when solving sudokus. It doesn't matter to me if I take 15 minutes or 2 hours to solve, as long as I solve it. I am currently working on a monstrous sudoku with 25 interlocking grids where there is a 26th within which is clueless. I first started working on it a couple of years ago and I work on it off and on, mostly off. Still not even close to finishing it.

@@antono.8896 true, it's about breaking a code or a safe by using a pattern, like discovering a special way to the source and not about trying every brutal and vapid cambination or hitting every road until you don't meet a dead end, it would be a really tiresome and boring trip for me this brutal force. And the worst is that you don't learn anything else ... is just the same "brutal pattern" on an on. Not here to say how to play sudoku, as long as you like it, enjoy it. I myself enjoy the hidden beauty of it.