Normal sudoku rules apply. Digits along a thermometer increase from the bulb end. A digit in a white circle indicates how many times that digit appears in white circles. If the same digit appears in the same position in two 3x3 boxes, that digit also appears in the same position in one more box. Eg if r1c3 and r4c6 are both 5 then another 3x3 box must contain a 5 in its top right hand corner cell.