In recreational mathematics, a polyform is a plane figure constructed by joining together identical basic polygons. The basic polygon is often (but not necessarily) a convex plane-filling polygon, such as a square or a triangle. More specific names have been given to polyforms resulting from specific basic polygons, as detailed in the table below. For example, a square basic polygon results in the well-known polyominoes.
The rules for joining the polygons together may vary, and must therefore be stated for each distinct type of polyform. Generally, however, the following rules apply:
- Two basic polygons may be joined only along a common edge.
- No two basic polygons may overlap.
- A polyform must be connected (that is, all one piece; see connected graph, connected space). Configurations of disconnected basic polygons do not qualify as polyforms.
- The mirror image of an asymmetric polyform is not considered a distinct polyform (polyforms are "double sided").
One can allow more than one basic polygon. The possibilities are so numerous that the exercise seems pointless, unless extra requirements are brought in. For example, the Penrose tiles define extra rules for joining edges, resulting in interesting polyforms with a kind of pentagonal symmetry.
Types and applications
Polyforms are a rich source of problems, puzzles and games. The basic combinatorial problem is counting the number of different polyforms, given the basic polygon and the construction rules, as a function of n, the number of basic polygons in the polyform. Well-known puzzles include the pentomino puzzle and the Soma cube.
Basic polygon (monoform) Polyform Applications line segment polystick square polyomino pentomino puzzle, Lonpos puzzle, Sudoku equilateral triangle polyiamond 30°-60°-90° triangle polydrafter Eternity puzzle right isosceles (45°-45°-90°) triangle polyabolo regular hexagon polyhex cube polycube Soma cube square polyominoid