Consider a rectangle partitioned into smaller rectangles such that the sides of each smaller rectangle is parallel to one of the sides of the original rectangle. Call the corners of each rectangle a vertex. For any segment joining two vertices, call it basic if no other vertex lie on it. (The segments must be part of the partitioning.) Find the maximum/minimum possible number of basic segments over all possible partitions of .
Solution
Consider a rectangle partitioned into smaller rectangles such that the sides of each smaller rectangle are parallel to one of the sides of the original rectangle. We aim to find the maximum and minimum possible number of basic segments over all possible partitions of .
Let be the number of vertices which are intersections of segments in the partition. Let be the number of basic segments in the partition. Let and be the number of vertical and horizontal interior lines, respectively, which contain a segment in the partition.
Clearly, , representing the four corners of . Each vertex which is an intersection of segments belongs to basic segments. Also, every basic segment belongs to two vertices. Hence,
Each vertex which is an intersection of segments belongs to rectangles, where respectively. Also, every rectangle belongs to four vertices. Hence,
which simplifies to
Now, subtracting twice equation (2) from equation (1), we get:
which simplifies to
From equation (2), we obtain that . Hence,
The maximum of is achieved, for example, when is partitioned into rectangles. Hence, the maximum number of basic segments is .
If we draw an extension of every interior segment until it meets the boundary of , we get a new partition into rectangles, and we clearly increase the total number of rectangles. Hence,
Also, if we extend every interior segment as far as possible along borders between rectangles, we finish at two vertices which are intersections of edges. All these endpoints are clearly distinct. Hence,
Using equations (3), (4), (5), and applying the AM-GM inequality, we get:
which implies
thus,
Therefore,
The minimum of is achieved, for example, when is partitioned into rectangles. Hence, the minimum number of basic segments is .
The answer is: .