This Sudoku has the twin distinctions of being rated "extreme," the most difficult type, and of having only 17 "givens" or starting numbers, the minimum possible for a unique solution. But it can be solved using the methods illustrated on this channel.
The 17 givens yield a dozen other numbers, though the tactics can be quite involved, as the video shows. After that, I hit an impasse. I decided to see if it was possible to place a number in a cell in such a way as to place that same number uniquely in all of the blocks where it was still absent. (I explain this strategy in videos 8 and 9 on this channel.) The only number that was possible for was the 2. There were actually two different ways to generate 2s in all the blocks: by putting a 2 in row 9, column 6 or in row 8, column 9. The first way generated a few other numbers after all the 2s, but then it hit another impasse. However, the second way, shown in the video, generated 20 numbers after the 2s.
Then there was another impasse. But I saw a way to generate 7s in all the blocks where they were still absent by supplying a 7 in a certain cell (row 3, column 6). And from there, I was able to fill in the rest of the puzzle.
On the one hand, there was no definitive reason why generating the 2s and 7s in this way should have solved the puzzle. Conceivably, the solution could have involved those numbers being in locations where they could not have all been generated at once. But on the other hand, a 17-given puzzle is so sparse that the numbers have to come from somewhere. It is reasonable to expect that the puzzle may be "programmed" to generate many numbers from a single one. So I would say that this is a technique that solvers should keep in their back pockets for situations such as this one in which it has logical, reasonable prospects of success.
My understanding is that this Sudoku is in the public domain.
5 ноя 2023
