In the geometry of tessellations, a rep-tile or reptile is a shape that can be dissected into smaller copies of the same shape. The term was coined as a pun on animal reptiles by the American mathematician Solomon W. Golomb, who used it to describe self-replicating tilings. In 2012 a generalization of rep-tiles called self-tiling tile sets was introduced by Lee Sallows in Mathematics Magazine.
- 1 Terminology
- 2 Examples
- 3 Rep-tiles and symmetry
- 4 Rep-tiles and polyforms
- 5 Pentagonal rep-tiles
- 6 Rep-tiles and fractals
- 7 Rep-tiles with multiple rep-tilings
- 8 Infinite tiling
- 9 See also
- 10 Notes
- 11 References
- 12 External links
A rep-tile is labelled rep-n if the dissection uses n copies. Such a shape necessarily forms the prototile for a tiling of the plane, in many cases an aperiodic tiling. A rep-tile dissection using different sizes of the original shape is called an irregular rep-tile or irreptile. If the dissection uses n copies, the shape is said to be irrep-n. If all these sub-tiles are of different sizes then the tiling is additionally described as perfect. A shape that is rep-n or irrep-n is trivially also irrep-(kn − k + n) for any k > 1, by replacing the smallest tile in the rep-n dissection by n even smaller tiles. The order of a shape, whether using rep-tiles or irrep-tiles is the smallest possible number of tiles which will suffice.
Every square, rectangle, parallelogram, rhombus, or triangle is rep-4. The sphinx hexiamond (illustrated above) is rep-4 and rep-9, and is one of few known self-replicating pentagons. The Gosper island is rep-7. The Koch snowflake is irrep-7: six small snowflakes of the same size, together with another snowflake with three times the area of the smaller ones, can combine to form a single larger snowflake.
A right triangle with side lengths in the ratio 1:2 is rep-5, and its rep-5 dissection forms the basis of the aperiodic pinwheel tiling. By Pythagoras' theorem, the hypotenuse, or sloping side of the rep-5 triangle, has a length of √5.
The international standard ISO 216 defines sizes of paper sheets using the √, in which the long side of a rectangular sheet of paper is the square root of two times the short side of the paper. Rectangles in this shape are rep-2. A rectangle (or parallelogram) is rep-n if its aspect ratio is √n:1. An isosceles right triangle is also rep-2.
Rep-tiles and symmetry
Some rep-tiles, like the square and equilateral triangle, are symmetrical and remain identical when reflected in a mirror. Others, like the sphinx, are asymmetrical and exist in two distinct forms related by mirror-reflection. Dissection of the sphinx and some other asymmetric rep-tiles requires use of both the original shape and its mirror-image.
Rep-tiles and polyforms
If a polyomino is rectifiable, or able to tile a rectangle, then it will also be a rep-tile, because the rectangle can be used to tile a square (which is itself a special case of the rectangle). This can be seen clearly in the octominoes, which are created from eight squares. Two copies of some octominoes will tile a square, therefore these octominoes are also rep-16 rep-tiles.
Similarly, if a polyiamond tiles an equilateral triangle, it will also be a rep-tile.
Polyforms based on isosceles right triangles, with angles 45°-90°-45°, are known as polyabolos. An infinite number of them are rep-tiles. Indeed, the simplest of all rep-tiles is a single isosceles right triangle. It is rep-2 when divided by a single line bisecting the right angle to the hypotenuse. Rep-2 rep-tiles are also rep-2n and the rep-4,8,16+ triangles yield further rep-tiles. These are found by discarding half of the sub-copies and permutating the remainder until they are mirror-symmetrical within a right triangle. In other words, two copies will tile a right triangle. One of these new rep-tiles is reminiscent of the fish formed from three equilateral triangles.
Triangular and quadrilateral (four-sided) rep-tiles are common, but pentagonal rep-tiles are rare. For a long time, the sphinx was widely believed to be the only example known, but the German/New-Zealand mathematician Karl Scherer and the American mathematician George Sicherman have found more examples, including a double-pyramid and an elongated version of the sphinx. These pentagonal rep-tiles are illustrated on the Math Magic pages overseen by the American mathematician Erich Friedman. However, the sphinx and its extended versions are the only known pentagons that can be rep-tiled with equal copies. See Clarke's Reptile pages.
Rep-tiles and fractals
Rep-tiles as fractals
Rep-tiles can be used to create fractals, or shapes that are self-similar at smaller and smaller scales. A rep-tile fractal is formed by subdividing the rep-tile, removing one or more copies of the subdivided shape, and then continuing recursively. For instance, the Sierpinski carpet is formed in this way from a rep-tiling of a square into 27 smaller squares, and the Sierpinski triangle is formed from a rep-tiling of an equilateral triangle into four smaller triangles. When one sub-copy is discarded, a rep-4 L-triomino can be used to create four fractals, two of which are identical except for orientation.
Fractals as rep-tiles
Because fractals are self-similar on smaller and smaller scales, many are also self-tiling and are therefore rep-tiles. For example, the Sierpinski triangle is rep-3, tiled with three copies of itself, and the Sierpinski carpet is rep-8, tiled with eight copies of itself.
Rep-tiles with multiple rep-tilings
Many of the common rep-tiles are rep-n2 for all positive integer values of n. In particular this is true for three trapezoids including the one formed from three equilateral triangles, for three axis-parallel hexagons (the L-tromino, L-tetromino, and P-pentomino), and the sphinx hexiamond.
Among regular polygons, only the triangle and square can be dissected into smaller equally sized copies of themselves. However, a regular hexagon can be dissected into six equilateral triangles, each of which can be dissected into a regular hexagon and three more equilateral triangles. This is the basis for an infinite tiling of the hexagon with hexagons. The hexagon is therefore an irrep-∞ or irrep-infinity irreptile.
- Gardner, M. (2001), "Rep-Tiles", The Colossal Book of Mathematics: Classic Puzzles, Paradoxes, and Problems, New York: W. W. Norton, pp. 46–58.
- Gardner, M. "Rep-Tiles: Replicating Figures on the Plane." Ch. 19 in The Unexpected Hanging and Other Mathematical Diversions. Chicago, IL: Chicago University Press, pp. 222–233, 1991.
- Langford, C. D. "Uses of a Geometric Puzzle." Math. Gaz., No. 260, 1940.
- Niţică, Viorel (2003), "Rep-tiles revisited", MASS selecta, Providence, RI: American Mathematical Society, pp. 205–217, MR 2027179.
- Sallows, Lee (2012), "On self-tiling tile sets", Mathematics Magazine, 85 (5): 323–333, doi:10.4169/math.mag.85.5.323, MR 3007213.
- Scherer, Karl. "A Puzzling Journey to the Reptiles and Related Animals”, 1987
- Wells, D. The Penguin Dictionary of Curious and Interesting Geometry. London: Penguin, pp. 213–214, 1991.
|Wikimedia Commons has media related to Rep-tiles.|
- Mathematics Centre Sphinx Album: http://mathematicscentre.com/taskcentre/sphinx.htm
- Clarke, A. L. "Reptiles." http://www.recmath.com/PolyPages/PolyPages/Reptiles.htm.
- Weisstein, Eric W. "Rep-Tile". MathWorld.
- http://www.uwgb.edu/dutchs/symmetry/reptile1.htm (1999)
- IFStile - program for finding rep-tiles: https://ifstile.com