27. coloin: You can always find new challenges in the world of Sudokus

It’s always possible to find new challenges in the world of Sudokus, and that's what this puzzle made possible. I explained in my comments on Imam bayildi (video #26) that I had stumbled onto a convention of the time when the ultra-difficult puzzles I have been exploring in recent videos were developed. (They all come from a thread in the Enjoy Sudoku forum, “Hardest Sudokus,” that was updated through 2011. There is a successor thread that I plan to explore once I have worked on all the puzzles in that one.) That convention was to put the numbers 1-9 in order across the top row. There was no real significance to that; those digits simply represented the unique values in those cells and their other occurrences throughout the puzzle. (Any Sudoku could be adapted to read this way; it would only be necessary to replace whatever number was in row 1, column 1 [r1c1] with a 1, there and in all of its other occurrences; to replace the number in r1c2 with a 2; and so forth.) It seems that these puzzles were designed to test and improve computer algorithms, and the algorithms would not have filled in the whole top row upon “seeing” on a handful of numbers in telltale positions.
But humans are different. It’s impossible for us to un-see something. Once you know this convention, as soon as you notice even one number “in position” and nothing preventing the other numbers from being in sequence across the top row (as in the case of this puzzle), it’s inevitable to consider whether that convention is in place. You think about its implications even if you don’t fill in the numbers. And if the convention actually is in place, that unfortunately keeps you from carrying out the kind of strategic analysis that you would otherwise.
This is a lot like what happened when I kept realizing that other ultra-difficult Sudokus had the same basic design as Platinum Blonde (video #14). I did not have to do the same work of overall analysis for those puzzles that I would have had to do without this realization. (Those puzzles were Golden Nugget, #15; Red Dwarf, #16; coly013, #17; tarx0134, #18; and Discrepancy, #24.)
So in this case I modified the challenge. I put the numbers 1-9 across the top row and then decided to see what interesting path I might follow through the puzzle on my way to finding out whether they all actually belonged there. As the video shows, once I got all the values I could in the top row of blocks, I tried to fill in the rest of the puzzle one row at a time. This required tactics at various points that I hope will be of interest and use to viewers.
In the video, at 00:30, I identify a cell that has to be either a 3 or a 9. As I explain in the information about this channel, these videos are about how to solve Sudokus using imagination and intuition as well as logic and analysis. And that is what I did here. I chose the 9 intuitively. That was because it felt solid and promising, while the 3 didn’t feel as if it would go anywhere. It turned out that the 9 led all the way to the finish. (You can think of it as the start of a forcing chain that ended up having 43 links.) As I have said several times in comments on these videos, I understand and respect the desire of many people to solve Sudokus without supplying numbers and without backtracking. But here I show another way to experience enjoyment and success with Sudokus.
There was a respected forum member, active at the time when these ultra-difficult puzzles were developed, who went by the name coloin. He was the creator of Platinum Blonde (video #14). But this Sudoku, even though it is called coloin, was apparently created by a different designer, one who went by the name eleven. Perhaps eleven created it as a challenge for or tribute to coloin. I'm not sure. But whatever was going on in the Sudoku forums in those days certainly generated a lot of amazing puzzles!