Triple Double Uniqueness

[@SudokuGuy]

Well done. I call this a Danger Danger Danger!!!

[@liezl713]

I solved this easy peasy with no special techniques. After the basics, I simply used the three cells in Row 9 in that first block to be able to pencil in the 2, 3, and 9 in the other 2 blocks next to it and then filled in the rest of that row, then it forms 2 pairs, (4/6) and (5/8). After that, the rest of it solves easy.

[@ShannonSettles]

At the point you start trying to figure this triple-uniqueness you have a much easier solution, an X-wing on 3s in column 2 and 3. It's a straightforward solve from there.

[@JohnRandomness105]

Now that you mention it, I can see the X-wing. Maybe Mark ignored it in favor of illustrating the uniqueness principle.

[@LinXiaoChaun]

It appears that this puzzle can be solved completely without recourse to any but the basic rules of sudoku. Just saying.

[@JohnRandomness105]

0:40 I just did the puzzle. It was straightforward, without requiring any chaining, or recognition of positions. Maybe it helped that I did it on SudokuExchange four years later. One advantage that engine has is that whenever I place a digit, all penciled digits seeing that digit are erased. I can see to click on a cell with only one center digit, or a single corner digit in a block. After watching the video, all I can say is that the ability to pencilmark made a major difference in this puzzle.

[@debralegorreta1375]

Yes. This is an extension of the BUG principle. If you hadn't put the 2 in C1R3, it would've had to go with the 3 you highlighted, making it a double; and that would've forced a 2-3 double in R9, a violation of BUG, or a 9-3 double in R9, the triple double you are talking about. Then this swordfish is self-contained: flipping one double will flip the other two yielding a second solution to the same puzzle, which is not allowed by BUG. There's yet another instance of the BUG principle in Columns 5, 9 and Rows 7,8, 9. Look first at Column 9, where the numbers 2, 5, 8 form a triple. The very same triple is threatening to form in Column 5. BUG then tells us 2 cannot be allowed to do that and therefore in Column 5 the 2 is restricted to Rows 1,2,3. This forces a 3 in R3C4. From there we get to where you got and finish the puzzle.

[@rabidsamfan]

I think of this as a variation on uniqueness, and yes, I've used it fairly often. Not quite in the same way -- in this case I would have used it first to place the 2 in top left square, thereby forcing the position of the 1, and then repeated the logic to place the 9. You jumped right to seeing that the 7 would have to be in one of the cells with the 3 next to it. I call it a zone defense (if three squares/boxes of nine across or down are a zone, then this only works within a zone.) It's an easy pattern to spot -- much easier than x wings etc. -- because it invariably involves at least one set of twins within a chute (the three space intersection of a square and a row or column). If you have twins a/b and a/b, and a parallel pair of a's in the next square over, it's worth looking at the third square to see if you have a parallel pair of b's. That way if there's a c lurking about trying to fit in you know that you have to kick one of them to the curb. It doesn't work in a sudoku variant if any of the spaces is affected by an extra rule, though. I don't think its dull at all, though. I always feel pretty smug when I can use it!

[@topilinkala1594]

I solved this by noticing after I had "penciled" the threes on row 2 that the middle of the second square is then a free 2. And that busted the whole problem.

[@ivanckuza3243]

The number 3 in Box 1 just in midle and right. So in Box 2 in the midle its hiden single.

[@goldenera7090]

wow that's really innovative.. true genius