Sierpinski triangle
Generated using a random algorithm
The Sierpinski triangle (also with the original orthography Sierpiński), also called the Sierpinski gasket or the Sierpinski Sieve, is a fractal and 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 selfsimilar sets, i.e. it is a mathematically generated pattern that can be reproducible at any magnification or reduction. It is named after the Polish mathematician Wacław Sierpiński, but appeared as a decorative pattern many centuries prior to 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:

Start with an equilateral triangle.

Subdivide it into four smaller congruent equilateral triangles and remove the central one.

Repeat step 2 with each of the remaining smaller triangles

Each removed triangle (a trema) is topologically an open set.^{[1]} 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:

Start with any triangle in a plane (any closed, bounded region in the plane will actually work). The canonical Sierpinski triangle uses an equilateral triangle with a base parallel to the horizontal axis (first image).

Shrink the triangle to ½ height and ½ width, make three copies, and position the three shrunken triangles so that each triangle touches the two other triangles at a corner (image 2). Note the emergence of the central hole  because the three shrunken triangles can between them cover only 3/4 of the area of the original. (Holes are an important feature of Sierpinski's triangle.)

Repeat step 2 with each of the smaller triangles (image 3 and so on).
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 "Vvariable fractals and superfractals."^{[2]}

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_a note the dilation by a factor of ½ about a point a, then the Sierpinski triangle with corners a, b, and c is the fixed set of the transformation d_a U d_b U d_c.
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_a, d_b, and d_c 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_{1}, p_{2} and p_{3} as the corners of the Sierpinski triangle, and a random point v_{1}. Set v_{n+1} = ½ ( v_{n} + p_{rn} ), where r_{n} is a random number 1, 2 or 3. Draw the points v_{1} to v_{∞}. If the first point v_{1} was a point on the Sierpiński triangle, then all the points v_{n} lie on the Sierpinski triangle. If the first point v_{1} to lie within the perimeter of the triangle is not a point on the Sierpinski triangle, none of the points v_{n} will lie on the Sierpinski triangle, however they will converge on the triangle. If v_{1} is outside the triangle, the only way v_{n} will land on the actual triangle, is if v_{n} is on what would be part of the triangle, if the triangle was infinitely large.
Animated creation of a Sierpinski triangle using the chaos game
Animated construction of a Sierpinski triangle
Or more simply:

Take 3 points in a plane to form a triangle, you need not draw it.

Randomly select any point inside the triangle and consider that your current position.

Randomly select any one of the 3 vertex points.

Move half the distance from your current position to the selected vertex.

Plot the current position.

Repeat from step 3.
Note: 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. It is interesting to do this with pencil and paper. A brief outline is formed after placing approximately one hundred points, and detail begins to appear after a few hundred.
Sierpinski triangle using an iterated function system
Arrowhead curve
Another construction for the Sierpinski triangle 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:

Start with a single line segment in the plane

Repeatedly replace each line segment of the curve with three shorter segments, forming 120° angles at each junction between two consecutive segments, with the first and last segments of the curve either parallel to the original line segment or forming a 60° angle with it.
The resulting fractal curve is called the Sierpiński arrowhead curve, and its limiting shape is the Sierpinski triangle.^{[3]}
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 lifelike cellular automaton automaton B1/S12 when applied to a single cell will generate four approximations of the Sierpinski triangle.^{[4]} The timespace diagram of a replicator pattern in a cellular automaton also often resembles a Sierpinski triangle.^{[5]}
Pascal's triangle
If one takes Pascal's triangle with 2^{n} 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 n approaches infinity of this paritycolored 2^{n}row Pascal triangle is the Sierpinski triangle.^{[6]}
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 ndisk puzzle, and the allowable moves from one state to another, form an undirected graph that can be represented geometrically as the intersection graph of the set of triangles remaining after the nth step in the construction of the Sierpinski triangle. Thus, in the limit as n goes to infinity, this sequence of graphs can be interpreted as a discrete analogue of the Sierpinski triangle.^{[7]}
Properties
For integer number of dimensions d, when doubling a side of an object, 2 ^{d} copies of it are created, i.e. 2 copies for 1dimensional object, 4 copies for 2dimensional object and 8 copies for 3dimensional object. For Sierpinski triangle doubling its side creates 3 copies of itself. Thus Sierpinski triangle has Hausdorff dimension log(3)/log(2) ≈ 1.585, which follows from solving 2 ^{d} = 3 for d.^{[8]}
The area of a Sierpinski triangle is zero (in Lebesgue measure). The area remaining after each iteration is clearly 3/4 of the area from the previous iteration, and an infinite number of iterations results in zero.^{[9]}
The points of a Sierpinski triangle have a simple characterization in Barycentric coordinates.^{[10]} If a point has coordinates (0.u_{1}u_{2}u_{3}…,0.v_{1}v_{2}v_{3}…,0.w_{1}w_{2}w_{3}…), expressed as Binary numbers, then the point is in Sierpinski's triangle if and only if u_{i}+v_{i}+w_{i}=1 for all i.
Analogues in higher dimensions
A Sierpinski squarebased pyramid and its 'inverse'
A Sierpiński trianglebased pyramid as seen from above (4 main sections highlighted). Note the selfsimilarity in this 2dimensional projected view, so that the resulting triangle could be a 2D fractal in itself.
The tetrix is the threedimensional 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. This can also be done with a square pyramid and five copies instead. A tetrix constructed from an initial tetrahedron of sidelength L has the property that the total surface area remains constant with each iteration.
The initial surface area of the (iteration0) tetrahedron of sidelength L is L^2 \sqrt{3}. At the next iteration, the sidelength is halved

L \rightarrow { L \over 2 }
and there are 4 such smaller tetrahedra. Therefore, the total surface area after the first iteration is:

4 \left( \left( {L \over 2} \right)^2 \sqrt{3} \right) = 4 { {L^2} \over 4 } \sqrt{3} = L^2 \sqrt{3}.
This remains the case after each iteration. Though the surface area of each subsequent tetrahedron is 1/4 that of the tetrahedron in the previous iteration, there are 4 times as many—thus maintaining a constant total surface area.
The total enclosed volume, however, is geometrically decreasing (factor of 0.5) with each iteration and asymptotically approaches 0 as the number of iterations increases. In fact, it can be shown that, while having fixed area, it has no 3dimensional character. The Hausdorff dimension of such a construction is \textstyle\frac{\ln 4}{\ln 2}=2 which agrees with the finite area of the figure. (A Hausdorff dimension strictly between 2 and 3 would indicate 0 volume and infinite area.)
History
Wacław Sierpiński described the Sierpinski triangle in 1915. However, similar patterns appear already in the 13thcentury Cosmati mosaics in the cathedral of Anagni, Italy,^{[11]} and other places of central Italy, for carpets in many places such as the nave of the Roman Basilica of Santa Maria in Cosmedin,^{[12]} and for isolated triangles positioned in rotae in several churches and Basiliche.^{[13]} In the case of the isolated triangle, it is interesting to notice that the iteration is at least of three levels.
See also
References

^ "Sierpinski Gasket by Trema Removal"

^ Michael Barnsley, et al."Vvariable fractals and superfractals" PDF (2.22 MB)

^ Prusinkiewicz, P. (1986), "Graphical applications of L−systems", Proceedings of Graphics Interface '86 / Vision Interface '86, pp. 247–253 .

^ Rumpf, Thomas (2010), "Conway's Game of Life accelerated with OpenCL", Proceedings of the Eleventh International Conference on Membrane Computing (CMC 11), pp. 459–462 .

^ Bilotta, Eleonora; Pantano, Pietro (Summer 2005), "Emergent patterning phenomena in 2D cellular automata", Artificial Life 11 (3): 339–362, .

^ Stewart, Ian (2006), How to Cut a Cake: And other mathematical conundrums, Oxford University Press, p. 145, .

^ Romik, Dan (2006), "Shortest paths in the Tower of Hanoi graph and finite automata", SIAM Journal on Discrete Mathematics 20 (3): 610–622 (electronic), .

^ Falconer, Kenneth (1990). Fractal geometry: mathematical foundations and applications. Chichester: John Wiley. p. 120.

^ Helmberg, Gilbert (2007), Getting Acquainted with Fractals, Walter de Gruyter, p. 41, .

^ http://www.cuttheknot.org/ctk/Sierpinski.shtml

^

^ "Geometric floor mosaic (Sierpinski triangles), nave of Santa Maria in Cosmedin, Forum Boarium, Rome", 5 September 2011, Flickr

^ Tedeschini Lalli, Elisa (2011),
External links

Weisstein, Eric W., "Sierpinski Sieve", MathWorld.

Paul W. K. Rothemund, Nick Papadakis, and Erik Winfree, Algorithmic SelfAssembly of DNA Sierpinski Triangles, PLoS Biology, volume 2, issue 12, 2004.

Sierpinski Gasket by Trema Removal at cuttheknot

Sierpinski Gasket and Tower of Hanoi at cuttheknot

3D printed Stage 5 Sierpinski Tetrahedron
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