Logic Masters Deutschland e.V.

Previous Dutch Flat Mate Puzzles
This has been a weekly series since '18 Oct 23'. Here is a spreadsheet link for all the previous puzzles in the series, which also has a few teasers for upcoming constraints/guest setters in the series (subject to change).
This alternative link offers a limited LMD search for Dutch Flat Mates.

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Special Thanks
I believe jovi_al was the first to use Internal X-Sums, and here are a selection of puzzles which use the constraint:

• 2/5 - Corners are Cornering (6x6) by Flinty :)
• 2/5 - Internal X-Sums Chaos Construction (6x6) by jovi_al
• 3/5 - Internal X-Sums Chaos Construction (9x9) by jovi_al
• 5/5 - Chaos Construction: Internal X-Sums by KNT

These are the current LMD difficulty ratings. The 6x6 are far from trivial, and I think you could make a strong case they are both closer to 3/5. I don't think KNT's would be considered a 5/5 difficulty today, and you certainly shouldn't be dissuaded by that if you enjoy this constraint, it's also lovely!

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To play the puzzle, please click the image:

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Puzzle Link: Click Here

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Initial Setter Difficulty: 3.5 / 5

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Chaos Construction
Normal Chaos Construction rules apply: Divide the grid into nine regions of nine orthogonally connected cells, and fill every cell with a digit from 1-9 so there aren't repeats in a row, column, or region.

Dutch Flats
Every 5 in the grid must have a '1' directly above it or a '9' directly below it. It may have both, but it doesn't need both.

Internal X-Sums
Digit X in a circle indicates that cell sees exactly X digits orthogonally in its region (including itself) and those digits sum to the value in the top left corner (if given). Region borders obstruct vision.

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Hint to start

The shape of the 45 clue is completely forced, as is the digit inside the circle. You need to consider region building, you can't grow into multiple clues that would contradict, and, —most importantly— keeping 5 happy! :)

Later Step Hint(s)

There is a lot of progress to be made on the left, with those clues being easier than the break-in for region building (the R4C3 '21' sum, which was essential for the break in, doesn't resolve cleanly until much later on). The trickiest deductions are Row 7 on the right. First you need to prove that R7C7 and R7C8 cannot be in the same region, and then you should be able to appreciate how they guide each other. A trickier question then, is can R7C8 and R7C9 be in the same region? Which resolves the R7C8 clue. If all else fails, recheck clues, consider tricky digit availability (such as 3s at the top!) and happy 5s of course! :)

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Next Week
The Dutch Flat Mates series welcomes another A-Tier setter making his debut! Richard, who you will surely know from his monumental "Sudoku Variant Series", will likely be the final debutant - and what a fitting end that would be. Together, we had an idea for adapting an exsiting constraint of his, which Richard has truly made his own. It debuted in 'vanilla' form as Sudoku Variants Series (484) - Next to N, and you're in for another treat when he adds happy 5s into the mix!