Logic Masters Deutschland e.V.

Links:

F-puzzles
CTC app

Rules:

Normal Sudoku rules apply.
The digit in the cell with the grey square is even.

Clues outside the grid represent cyclical sandwich clues. A valid cyclical sandwich clue is as follows:

For horizontal clues, start in the first cell next to the clue in column 1, then go to the column indexed by that cell, Then go to the column indexed by that cell. Repeat this process until you return to column 1. The clue is the sum of all of the cells traversed in this process except the 1 and the 9. The 1 and the 9 must be part of the cycle for the clue to be valid.

Vertical clues are identical except each cell indexes the row of the next cell starting with the cell next to the clue in row 1. Similarly, the 1 and the 9 must be part of the cycle for the clue to be valid.

Example and 6x6:

For a concrete example, if r1c1 was 2, r1c2 was 9, r1c9 was 8, and r1c8 was 1, this would be a valid horizontal 10 clue. For an invalid example, if r1c5 is 4, an r1C4 is 1, this would not be a valid 4 clue as it does not contain a 9.

You can also see below for a valid 6x6 example (replacing 6 for 9 as the 'crust' digit) . This has a logical solve path which you can try on F-puzzles or the CTC app

For instance, in the below 6x6, the 3 clue in row 3 and the 3 clue in column 3 both go from 3 to 6 to 1 (which goes back to 3). The vertical 14 clue goes from 2 -> 6 -> 5 -> 3 -> 4 -> 1

Solution guide

You can see a guide of the logic in this puzzle here . The guide has spoilers in it, so don't consult it if you'd like to solve the puzzle for yourself.