Sierpiński triangle
The Sierpiński triangle (sometimes spelled "Sierpinski"), also called the Sierpiński gasket or Sierpiński sieve, is a fractal attractive fixed set with the overall shape of an equilateral triangle, subdivided recursively into smaller equilateral triangles. Originally constructed as a curve, this is one of the basic examples of self-similar sets—that is, it is a mathematically generated pattern that is reproducible at any magnification or reduction. It is named after the Polish mathematician Wacław Sierpiński, but appeared as a decorative pattern many centuries before the work of Sierpiński. Constructions. There are many different ways of constructing the Sierpinski  triangle. Removing triangles. The Sierpinski triangle may be constructed from an equilateral triangle by repeated removal of triangular subsets: Each removed triangle (a "trema") is topologically an open set. This process of recursively removing triangles is an example of a finite subdivision rule. Shrinking and duplication. The same sequence of shapes, converging to the Sierpinski triangle, can alternatively be generated by the following steps: Note that this infinite process is not dependent upon the starting shape being a triangle—it is just clearer that way. The first few steps starting, for example, from a square also tend towards a Sierpinski  triangle. Michael Barnsley used an image of a fish to illustrate this in his paper "V-variable fractals and superfractals." The actual fractal is what would be obtained after an infinite number of iterations. More formally, one describes it in terms of functions on closed sets of points. If we let "d" denote the dilation by a factor of about a point A, then the Sierpinski triangle with corners A, B, and C is the fixed set of the transformation "d" ∪ "d" ∪ "d". This is an attractive fixed set, so that when the operation is applied to any  other set repeatedly, the images converge on the Sierpinski triangle. This is what is happening with the triangle above, but any other set would suffice. Chaos game. If one takes a point and applies each of the transformations "d", "d", and "d" to it randomly, the resulting points will be dense in the Sierpinski triangle, so the following algorithm will again generate arbitrarily close approximations to it: Start by labeling p, p and p as the corners of the Sierpinski triangle, and a random point v. Set , where "r" is a random number 1, 2 or 3. Draw the points  v to v. If the first point v was a point on the Sierpiński triangle, then all the points v lie on the Sierpinski triangle. If the first point v to lie within the perimeter of the triangle is not a point on the Sierpinski triangle, none of the points v will lie on the Sierpinski triangle, however they will converge on the triangle. If v is outside the triangle, the only way v will land on the actual triangle, is if v is on what would be part of the triangle, if the triangle was infinitely large. Or more  simply: This method is also called the chaos game, and is an example of an iterated function system. You can start from any point outside or inside the triangle, and it would eventually form the Sierpinski Gasket with a few leftover points (if the starting point lies on the outline of the triangle, there are no leftover points). With pencil and paper, a brief outline is formed after placing approximately one hundred points, and detail begins to appear after a few hundred. Arrowhead construction of Sierpinski gasket. Another construction for the Sierpinski gasket shows that it can be constructed as  a curve in the plane. It is formed by a process of repeated modification of simpler curves, analogous to the construction of the Koch snowflake: At every iteration, this construction gives a continuous curve. In the limit, these approach a curve that traces out the Sierpinski triangle by a single continuous directed (infinitely wiggly) path, which is called the Sierpinski arrowhead. In fact, the aim of the original article by Sierpinski of 1915, was to show an example of a curve (a Cantorian curve), as the title of the article itself declares. Cellular automata. The Sierpinski triangle also appears in  certain cellular automata (such as Rule 90), including those relating to Conway's Game of Life. For instance, the Life-like cellular automaton B1/S12 when applied to a single cell will generate four approximations of the Sierpinski triangle. A very long one cell thick line in standard life will create two mirrored Sierpinski triangles. The time-space diagram of a replicator pattern in a cellular automaton also often resembles a Sierpinski triangle, such as that of the common replicator in HighLife. The Sierpinski triangle can also be found in the Ulam-Warburton automaton and the Hex-Ulam-Warburton automaton . Pascal's triangle. If one takes Pascal's triangle with formula_1 rows and colors the even numbers white, and the odd numbers black, the result is an approximation to the Sierpinski triangle. More precisely, the limit as formula_2 approaches infinity of this parity-colored formula_1-row Pascal triangle is the Sierpinski triangle. Towers of Hanoi. The Towers of Hanoi puzzle involves moving disks of different sizes between three pegs, maintaining the property that no disk is ever placed on top of a smaller disk. The states of an formula_2-disk puzzle, and the allowable moves from one state to another , form an undirected graph, the Hanoi graph, that can be represented geometrically as the intersection graph of the set of triangles remaining after the formula_2th step in the construction of the Sierpinski triangle. Thus, in the limit as formula_2 goes to infinity, this sequence of graphs can be interpreted as a discrete analogue of the Sierpinski triangle. Properties. For integer number of dimensions formula_7, when doubling a side of an object, formula_8 copies of it are created, i.e. 2 copies for 1-dimensional object, 4 copies for 2-dimensional object and 8 copies for 3-dimensional object. For the Sierpinski  triangle, doubling its side creates 3 copies of itself. Thus the Sierpinski triangle has Hausdorff dimension formula_9, which follows from solving formula_10 for formula_7. The area of a Sierpinski triangle is zero (in Lebesgue measure). The area remaining after each iteration is formula_12 of the area from the previous iteration, and an infinite number of iterations results in an area approaching zero. The points of a Sierpinski triangle have a simple characterization in barycentric coordinates. If a point has barycentric coordinates formula_13, expressed as binary numerals, then the point is in Sierpinski's triangle if and only if formula_14 for  Generalization to other moduli. A generalization of the Sierpinski triangle can also be generated using Pascal's triangle if a different modulus formula_15 is used. Iteration formula_2 can be generated by taking a Pascal's triangle with formula_17 rows and coloring numbers by their value modulo formula_15. As formula_2 approaches infinity, a fractal is generated. The same fractal can be achieved by dividing a triangle into a tessellation of formula_20 similar triangles and removing the triangles that are upside-down from the original, then iterating this step with each smaller triangle. Conversely, the fractal can also be generated by beginning  with a triangle and duplicating it and arranging formula_21 of the new figures in the same orientation into a larger similar triangle with the vertices of the previous figures touching, then iterating that step. Analogues in higher dimensions. The Sierpinski tetrahedron or tetrix is the three-dimensional analogue of the Sierpinski triangle, formed by repeatedly shrinking a regular tetrahedron to one half its original height, putting together four copies of this tetrahedron with corners touching, and then repeating the process. A tetrix constructed from an initial tetrahedron of side-length formula_22 has the property that the total surface area remains  constant with each iteration. The initial surface area of the (iteration-0) tetrahedron of side-length formula_22 is formula_24. The next iteration consists of four copies with side length formula_25, so the total area is formula_26 again. Subsequent iterations again quadruple the number of copies and halve the side length, preserving the overall area. Meanwhile, the volume of the construction is halved at every step and therefore approaches zero. The limit of this process has neither volume nor surface but, like the Sierpinski gasket, is an intricately connected curve. Its Hausdorff dimension is formula_27; here "log" denotes the natural logarithm, the  numerator is the logarithm of the number of copies of the shape formed from each copy of the previous iteration, and the denominator is the logarithm of the factor by which these copies are scaled down from the previous iteration. If all points are projected onto a plane that is parallel to two of the outer edges, they exactly fill a square of side length formula_28 without overlap. History. Wacław Sierpiński described the Sierpinski triangle in 1915. However, similar patterns appear already as a common motif of 13th-century Cosmatesque inlay stonework. The Apollonian gasket was first described by Apollonius  of Perga (3rd century BC) and further analyzed by Gottfried Leibniz (17th century), and is a curved precursor of the 20th-century Sierpiński triangle. Etymology. The usage of the word "gasket" to refer to the Sierpinski triangle refers to gaskets such as are found in motors, and which sometimes feature a series of holes of decreasing size, similar to the fractal; this usage was coined by Benoit Mandelbrot, who thought the fractal looked similar to "the part that prevents leaks in motors".