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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