Logic Masters Deutschland e.V.

Place the digits 1-9 once into each row, column, and 3x3 box, and draw a loop through the centers of some cells. Each segment of the loop connects two orthogonally or diagonally adjacent cells. The loop cannot intersect itself, and must visit each box. Cells with circles are not on the loop; they contain the number of orthogonally and diagonally adjacent cells that are on the loop. A section of the loop is provided.

Every cell on the loop is both row-indexing and column-indexing, and each of the cells it points to must also be on the loop. I.e. if the cell at row R, column C contains digit D, and it is on the loop, then it points to row D, column C, which must contain the digit R (row-indexing), and it also points to row R, column D, which must contain digit C (column-indexing, AKA the traditional "159" behavior).

Online solver: SudokuPad