The minimum number of given numbers that produce a valid Sudoku with a unique solution has been proven to be $17$, so it got me thinking, what is the maximum number of given numbers that still produces a minimal Sudoku?
A minimal Sudoku puzzle is one where removing any of the given digits leads to multiple possible solutions.
From a source I found on the Wikipedia section for this problem. here's a minimal Sudoku with 40 clues:
It is not known whether 40 is optimal.