What is the least number of white knights that can be placed on a standard chessboard so that a black piece cannot be added to an unoccupied square without being under attack?
In my previous post, I demonstrated an example with 12 knights.
To see that 12 is optimal, note that no knight can cover (either occupy or attack) more than one square labeled x in the following diagram:
x x . . . . . x
. x . . . . x x
. . . . . . . .
. . . . . . . .
. . . . . . . .
. . . . . . . .
x x . . . . x .
x . . . . . x x
Thus, at least 12 knights are required, which proves that the answer is 12.
Complete Solution
