Normal sudoku rules apply. Adjacent digits on a turquoise line must have the same difference, though this difference can be different for different lines. Each quad (ie black) circle contains all the digits in the surrounding 4 cells. Unfortunately these quad clues have all been written in white text, and so are invisible. Digits in a grey circle must be odd. A digit in a circle (odd or quad clue) shows how many times that digit appears in a circle. E.g. if there is a 5 in a quad clue, then there must be 5 total appearances of 5s in quad clues and odd circles combined.