Normal sudoku rules apply. A digit inside a white circle must appear in one of the four cells surrounding that circle. Digits in columns 1, 5, and 9 indicate the column in which the digit 1, 5, or 9 (respectively) appears in that row. Eg if the r4c1 is a 6 then, in row 4, the 1 must appear in the 6th column (ie r4c6).