Logic Masters Deutschland e.V.

Friends: Green cells are friendly. The digit in a friendly cell must be that cell's row, column, or box number.

Benefits: Each friendly cell is also a triple indexing cell. If such a cell in row R, column C, and box position P contains digit N, then the cell in column N of that row contains C, the cell in row N of that column contains R and the cell in position N in that box contains P. (For instance if r3c5 contained a 2, r3c2 would contain 5, r2c5 would contain 3, and r1c5 would contain 8.)

Note that boxes and box positions are numbered left to right, then top to bottom.

f-puzzles: https://f-puzzles.com/?id=2ahf8ky3
CTC: https://tinyurl.com/3t9dne67