A discrete mathematics problem by karandeep singh ludhar
A 7 × 7 grid is given with its four corners deleted. All 45 squares are initially white. What is the smallest number of squares which should be colored black so that an uncolored (white) 5-square cross cannot be found?
Note: A 5-square cross is also known as the X-pentomino.
Note : Adapted from bulgaria olympiad.