I wonder if there are proper $9\times9$ Sudokus having $7$ or less different clues. I know that $17$ is the minimum number of clues. In most Sudokus there are $1$ to $4$ clues of every number. Sometimes I found a Sudoku with only $8$ different clues.
In this example the number $9$ is missing, but the Sudoku was very well solvable. Is it possible to have a $9\times9$ Sudoku with less than $8$ different clues?