Normal sudoku rules apply. Digits in a circle must appear at least once in the four cells touched by the circle. A digit in column 1 indicates the column in which the digit 1 appears in that row. Columns 5 and 9 have the same rule for the digits 5 and 9 respectively. (For example, an 8 in R7C1 tells you there's a 1 in R7C8. A 3 in R2C5 tells you there's a 5 in R2C3.)