Normal sudoku rules apply. Some circles contain a maths operator (+,-,x,/). If you apply that operation to each pair of diagonal digits touching the circle then you'll get the number in the circle as the result BOTH times. A circle with a ? instead of number indicates that the result is the same for both pairs of digits but must be deduced. If a circle does not contain an operator, the digit(s) in the cirlce must appear in at least one of the cells touching the circle.