Normal sudoku rules apply. The digits along each arrow must multiply to the digit in the circled cell. Nine cells in the grid (one per row, column and 3x3 box) are Doublers, which count as double their value on arrows. Each digit appears in exactly one Doubler. Doublers aren't necessarily on arrows. The value of the Doubler is counted if it appears on a one cell arrow. Eg if r7c6 is a 3 and a Doubler then r6c5 is a 6.