Combinatorics · Extremal combinatorics · Rooks on a board · Rows and columns
At most how many rooks can be placed on a chessboard of dimensions \(5 \times 4\) (five rows and four columns) so that every rook attacks at most one of the remaining ones?
Here a rook attacks every rook standing in the same row as itself, and every rook standing in the same column as itself.
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Serbian Regional Competition 2008, high school grade I, category A, problem 3. Organized by the Mathematical Society of Serbia (DMS). Source