Normal sudoku rules apply. Box borders divide blue lines into segments with the same sum. Different blue lines can have different sums. If two digits (called x and y) are neighbors along a blue line, then x as well as y has to be an integer multiple of the difference between x and y. For example, 6 and 4 are allowed to be adjacent on a blue line since both 6 and 4 are a multiple of 2 (the difference between 4 and 6).