As a
quick reply to last posts comment :

Another
interesting question: How many clues (numbers allready in the grid) do
we need a Sudoku puzzle to have in the beginning in order to obtain a
unique solution? Comment by A.R.Ray

At
least one student proved that in micro-Sudoku (on a 4×4 grid)
one needs just 4 hints to get any unique solution (and that 4 is
minimal). It is an application of the fact that the micro-Sudoku group
acts on the set of all solutions with just two orbits so one needs to
check just these two solutions…

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