Problem:
A polyomino is a connected figure constructed by joining one or more unit squares edge-to-edge. Determine, with proof, the number of non-congruent polyominoes with no holes, perimeter , and area .
, 2025
Solution
Solution:
Define the bounding box of a polyomino to be the smallest axis-aligned rectangle that contains the entire polyomino. Suppose a polyomino satisfying the given conditions has a bounding box with dimensions .
Claim 1. .
Proof. The polyomino has at least horizontal edges and at least vertical edges. Moreover, it has a perimeter of . Therefore, , so .
Claim 2. The dimensions of the bounding box are either , , or .
Proof. Note that since it contains the polyomino with area . Suppose for sake of contradiction that . Then,
contradiction. Therefore, , so we can let . Then, implies that , as desired.
In the first and third cases, the bounding box has area , so it must be the entire polyomino, giving us the rectangle (and its rotation) as a possible answer. In the second case, the bounding box has area , so one cell must be removed to form the polyomino. Removing the corner cell yields a polyomino with perimeter , and removing any other cells yields a polyomino with perimeter greater than . Therefore, the only other possibility is a square missing a corner. Thus the answer is .