What is the minimum number of knights you need to place on a 9x9 chess board, such that every empty cell is attacked by at least one knight?

Here is a similar question for a 10x10 chess board: [Knights covering a 10x10 chess board](https://puzzling.stackexchange.com/questions/90169/knights-covering-a-10x10-chess-board/)

Hint:

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>  Even though the two puzzles are similar, their solutions are quite different. Furthermore, knowing the answer to one may prove that the answer to the other one is minimal.
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Good luck!