The construction of the Sierpinski carpet begins with a square. The square is cut into 9 congruent subsquares in a 3-by-3 grid, and the central subsquare is removed. The same procedure is then applied recursively to the remaining 8 subsquares, ad infinitum. It can be realised as the set of points in the unit square whose coordinates written in base three do not both have a digit '1' in the same position. The process of recursively removing squares is an example of a finite subdivision rule.
Properties
The area of the carpet is zero. The interior of the carpet is empty. The Hausdorff dimension of the carpet is. Sierpiński demonstrated that his carpet is a universal plane curve. That is: the Sierpinski carpet is a compact subset of the plane with Lebesgue covering dimension 1, and every subset of the plane with these properties is homeomorphic to some subset of the Sierpinski carpet. This 'universality' of the Sierpinski carpet is not a true universal property in the sense of category theory: it does not uniquely characterize this space up to homeomorphism. For example, the disjoint union of a Sierpinski carpet and a circle is also a universal plane curve. However, in 1958 Gordon Whyburn uniquely characterized the Sierpinski carpet as follows: any curve that is locally connected and has no 'local cut-points' is homeomorphic to the Sierpinski carpet. Here a local cut-point is a point for which some connected neighborhood of has the property that is not connected. So, for example, any point of the circle is a local cut point. In the same paper Whyburn gave another characterization of the Sierpinski carpet. Recall that a continuum is a nonempty connected compact metric space. Suppose is a continuum embedded in the plane. Suppose its complement in the plane has countably manyconnected components and suppose:
the diameter of goes to zero as ;
the boundary of and the boundary of are disjoint if ;
The topic of Brownian motion on the Sierpinski carpet has attracted interest in recent years. Martin Barlow and Richard Bass have shown that a random walk on the Sierpinski carpet diffuses at a slower rate than an unrestricted random walk in the plane. The latter reaches a mean distance proportional to after steps, but the random walk on the discrete Sierpinski carpet reaches only a mean distance proportional to for some. They also showed that this random walk satisfies stronger large deviation inequalities and that it satisfies the elliptic Harnack inequality without satisfying the parabolic one. The existence of such an example was an open problem for many years.
Wallis sieve
A variation of the Sierpinski carpet, called the Wallis sieve, starts in the same way, by subdividing the unit square into nine smaller squares and removing the middle of them. At the next level of subdivision, it subdivides each of the squares into 25 smaller squares and removes the middle one, and it continues at the th step by subdividing each square into smaller squares and removing the middle one. By the Wallis product, the area of the resulting set is, unlike the standard Sierpinski carpet which has zero limiting area. However, by the results of Whyburn mentioned above, we can see that the Wallis sieve is homeomorphic to the Sierpinski carpet. In particular, its interior is still empty.
Applications
Mobile phone and WiFi fractal antennas have been produced in the form of few iterations of the Sierpinski carpet. Due to their self-similarity and scale invariance, they easily accommodate multiple frequencies. They are also easy to fabricate and smaller than conventional antennas of similar performance, thus being optimal for pocket-sized mobile phones.
