As explained by Pettersen: "This is how: Let be the space of -grids built by legal sudoku bands, but with no attention put on whether the columns follow the rules of Sudoku. The size of is . Let also be the set of grids built by legal stacks with no attention put on the rows, is then . The nxm-sudoku grids are then the intersection of and . A random and are identical in a given box with probability , and under the assumption that these probabilities are independent for each box, we arrive at the estimate above."