It is known that the minimum number of clues a sudoku must have to have a unique solution is 17. But on every website I've seen for them, I haven't found any that are rotationally symmetrical.
I once saw a puzzle in a book which only had 19 clues and was rotationally symmetrical, but I don't remember whether the solver I ran it through said that there was a unique solution or not. My question is, what is the minimum number of already-filled-in squares that a rotationally symmetrical Sudoku must have in order to have a unique solution?
Note: by rotationally symmetrical, I'm referring to it in the standard sense that if a clue appears in one position, a clue will also appear in the position opposite it on the board.