pnm wrote:

As you can see, the 4x4 difficult there is quite tricky

marblevolcano wrote:

That is quite a tough puzzle as well, it seems. Well, everything is better in Japan

It's not that hard if you check sums in the right and left halves of the puzzle: [2,4] in the right "2–" in the lower row would mean 5 in "17+"'s leftmost cell (its "outie" to the left half), which is of course impossible. It must be [1,3] in the right "2–", 3 in the outie and, given [2,4] in the left "2–", 6 as the sum in "0–". Since the highest number in a "0–" is half the sum of all the numbers, it must contain [1,2,3] and the rest follows automatically.

Or you could start by checking the sum in the three upper rows to find that the sum in "0–" must be 3(10) – 2 – 17 – 5 = 6 and go on from there.

Anyway, what made this approach seem so obvious to me is probably the hefty dose of sum-only puzzles I'm currently getting from

the Colossal Killer Sudoku Book (much recommended, by the way)