Every 9x9 box is a valid sudoku, making 4 sudokus in all. Hopefully this one is a little more challenging than the previous one. I tried keeping the clues symmetric, but didn't quite manage. Enjoy!
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$\begingroup$ Looking nice! I'll leave the chance to others though. If nobody answers in a couple of hours, then I'll give a try. $\endgroup$– WhatsUpOct 15, 2020 at 20:17
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1 Answer
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The solution is
The first step is to notice something about the 3x3 squares on the outside. Specifically,
Each digit in one of those 12 squares will be in the same column and/or row as the same digit in the opposite square. For example, the 9 in the upper-left corner means that in the lower-left square the 9 must be in the left-most column. The 9 in the middle of the lower-right square means the 9 in the lower-left square must be in the middle row. Applying this logic to all the digits in the corners means it's pretty easy to solve the corners first.
Applying that logic gives us our first step:
From there, you can get the next little bit done with very basic Sudoku logic
This was about when I had to start writing down the possibilities in each square, but I never had to resort to any of the more advanced Sudoku solving techniques (I did use the technique I mentioned above). Here's what it looks like once you've got the middle four squares almost done.
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$\begingroup$ Nicely done! The problem with making them tougher to solve is to ensure they are still unique. Oh well... $\endgroup$– JensOct 16, 2020 at 15:10
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$\begingroup$ @Jens FYI by "advanced techniques" I mean things like the X-Wing. I'd rate this a 3/5 for difficulty, which I'd say is a good level for a Sudoku to be nontrivial but not so difficult that you have to scratch your head for a while just looking for where you can figure out another number. $\endgroup$ Oct 16, 2020 at 15:46