Scanraid rates this as a diabolical grade puzzle requiring all sorts of fiddly techniques to get through. But it is also rendered much easier by a quick application of the so-called Phistomefel's theorem. This, to the best of my knowledge, is first discussed here. It's also been picked up and popularised by various cracking the cryptic videos, which I am too lazy to link to but I'm sure all my dearest readers are either already familiar with or else can find for themselves.
For various reasons, Hu's puzzle does not seem to getting the attention I believe it deserves in the logic masters community. To me, this seems to be the first instance of a puzzle that really makes good use of the solving technique. Previous versions of puzzles that have attempted to incorporate it have always had equivalent shortcuts using much easier solving techniques, and people had speculated whether this was of any practical solving interest. Now we know!
Reading on in this thread I was pleasantly surprised to see a generalisation of the theory by Fred Stalder, which is well worth reading as well.
This will also help with another really remarkable puzzle, also courtesy of courtesy of 胡蒙汀(Hu Meng Ting). This one easily gets a 10/10 for difficulty, and maybe even an 11 for the sheer genius of the puzzle
This one is unsolvable by scanraid, and is certainly not easy even when you are armed with both the theory, and the equally remarkable thread of hints to get you going - there is a lot of work to do to get through this puzzle, but it is just about doable. I find this fascinating as what it really means is we are now shining a light on some of the hidden complexities underlying the structure of sudoku.
Anyhow, I am very grateful to cdwg2000 for sharing these puzzles and the theory and I shall certainly keep an eye out for anything else that they share, hoping that they haven't been discouraged too much.
All this talk of new and exciting theory has also made me a little bit nostalgic. I am going to offer up nothing related to this particular subject (although I'd invite my dearest readers to take another look at one of the classics on the 2020 edition of the UK Sudoku Championship if you're interested). Instead I feel like I've got a pretty nice execution of an idea that once upon a time was done to death. Enjoy!
#350 Sudoku – rated 3/10 [Easy]