Open set condition

an open set covering of the sierpinski triangle along with one of its mappings ψ_i.

In fractal geometry, the open set condition (OSC) is a commonly imposed condition on self-similar fractals. In some sense, the condition imposes restrictions on the overlap in a fractal construction. Specifically, given an iterated function system of contractive mappings ${\psi }_1,\dots ,{\psi }_m$, the open set condition requires that there exists a nonempty, open set V satisfying two conditions:

1. $\bigcup _{i=1}^m{\psi }_i(V)\subseteq V,$
2. The sets ${\psi }_1(V),\dots ,{\psi }_m(V)$ are pairwise disjoint.

Introduced in 1946 by P.A.P Moran, the open set condition is used to compute the dimensions of certain self-similar fractals, notably the Sierpinski Gasket. It is also used to simplify computation of the packing measure.

An equivalent statement of the open set condition is to require that the s-dimensional Hausdorff measure of the set is greater than zero.

Computing Hausdorff dimension

When the open set condition holds and each ${\psi }_i$ is a similitude (that is, a composition of an isometry and a dilation around some point), then the unique fixed point of $\psi$ is a set whose Hausdorff dimension is the unique solution for s of the following:

$\sum _{i=1}^m{r}_i^s=1.$

where r_i is the magnitude of the dilation of the similitude.

With this theorem, the Hausdorff dimension of the Sierpinski gasket can be calculated. Consider three non-collinear points a₁, a₂, a₃ in the plane R² and let ${\psi }_i$ be the dilation of ratio 1/2 around a_i. The unique non-empty fixed point of the corresponding mapping $\psi$ is a Sierpinski gasket, and the dimension s is the unique solution of

${\left(\frac{1}{2}\right)}^s+{\left(\frac{1}{2}\right)}^s+{\left(\frac{1}{2}\right)}^s=3{\left(\frac{1}{2}\right)}^s=1.$

Taking natural logarithms of both sides of the above equation, we can solve for s, that is: s = ln(3)/ln(2). The Sierpinski gasket is self-similar and satisfies the OSC.

Strong open set condition

The strong open set condition (SOSC) is an extension of the open set condition. A fractal F satisfies the SOSC if, in addition to satisfying the OSC, the intersection between F and the open set V is nonempty. The two conditions are equivalent for self-similar and self-conformal sets, but not for certain classes of other sets, such as function systems with infinite mappings and in non-euclidean metric spaces. In these cases, SOCS is indeed a stronger condition.

See also

• Cantor set
• List of fractals by Hausdorff dimension
• Minkowski–Bouligand dimension
• Packing dimension