Putnam 2016, Problem A-4

Consider a \( (2m - 1) \times (2n - 1) \) rectangular region, where \( m \) and \( n \) are integers such that \( m, n \geq 4 \). This region is to be tiled using tiles of the two types shown:
(The dotted lines divide the tiles into \( 1 \times 1 \) squares.) The tiles may be rotated and reflected, as long as their sides are parallel to the sides of the rectangular region. They must all fit within the region, and they must cover it completely without overlapping. What is the minimum number of tiles required to tile the region?

Solution:
The minimum number of tiles is $mn$. To see that this many are required, label the squares $(i,j)$ with $1 \leq i \leq 2m-1$ and $1 \leq j \leq 2n-1$, and for each square with $i,j$ both odd, color the square red; then no tile can cover more than one red square, and there are $mn$ red squares.

It remains to show that we can cover any $(2m-1) \times (2n-1)$ rectangle with $mn$ tiles when $m,n \geq 4$. First note that we can tile any $2 \times (2k-1)$ rectangle with $k \geq 3$ by $k$ tiles: one of the first type, then $k-2$ of the second type, and finally one of the first type. Thus if we can cover a $7 \times 7$ square with 16 tiles, then we can do the general $(2m-1) \times (2n-1)$ rectangle, by decomposing this rectangle into a $7 \times 7$ square in the lower left corner, along with $m-4$ $(2 \times 7)$ rectangles to the right of the square, and $n-4$ $((2m-1) \times 2)$ rectangles above, and tiling each of these rectangles separately, for a total of $16 + 4(m-4) + m(n-4) = mn$ tiles.

To cover the $7 \times 7$ square, note that the tiling must consist of 15 tiles of the first type and 1 of the second type, and that any $2 \times 3$ rectangle can be covered using 2 tiles of the first type. We may thus construct a suitable covering by covering all but the center square with eight $2 \times 3$ rectangles, in such a way that we can attach the center square to one of these rectangles to get a shape that can be covered by two tiles. An example of such a covering, with the remaining $2 \times 3$ rectangles left intact for visual clarity, is depicted below. (Many other solutions are possible.)