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").
Polyforms can also be considered in higher dimensions. In 3-dimensional space, basic polyhedra
can be joined along congruent faces. Joining cubes
in this way leads to the polycubes
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
. 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