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64 cubes, each with five white faces and one black face

Source: 1969 Hungary - Kürschák Competition p3

October 11, 2022
combinatoricsColoringcombinatorial geometry

Problem Statement

We are given 6464 cubes, each with five white faces and one black face. One cube is placed on each square of a chessboard, with its edges parallel to the sides of the board. We are allowed to rotate a complete row of cubes about the axis of symmetry running through the cubes or to rotate a complete column of cubes about the axis of symmetry running through the cubes. Show that by a sequence of such rotations we can always arrange that each cube has its black face uppermost