The statistician John Nelder died last Saturday.
Apart from many other achievements, he is the person with perhaps the best claim to have nearly invented Sudoku.
He defined a critical set in a Latin square to be a set of entries which can be completed in only one way. This is just a Sudoku puzzle, ignoring the 3×3 squares. (There is a small complication: a critical set should also be minimal, so that if you omit any of its elements, the solution is no longer unique; Sudoku does not usually require this.)
