Collage theorem

Let ${\displaystyle \mathbb {X} }$ be a complete metric space. Suppose ${\displaystyle L}$ is a nonempty, compact subset of ${\displaystyle \mathbb {X} }$ and let ${\displaystyle \epsilon >0}$ be given. Choose an iterated function system (IFS) ${\displaystyle \{\mathbb {X} ;w_{1},w_{2},\dots ,w_{N}\}}$ with contractivity factor ${\displaystyle 0\leq s<1}$, (The contractivity factor of the IFS is the maximum of the contractivity factors of the maps ${\displaystyle w_{i}}$.) Suppose

${\displaystyle h\left(L,\bigcup _{n=1}^{N}w_{n}(L)\right)\leq \varepsilon ,}$

where ${\displaystyle h(\cdot ,\cdot )}$ is the Hausdorff metric. Then

${\displaystyle h(L,A)\leq {\frac {\varepsilon }{1-s}}}$

where A is the attractor of the IFS. Equivalently,

${\displaystyle h(L,A)\leq (1-s)^{-1}h\left(L,\cup _{n=1}^{N}w_{n}(L)\right)\quad }$, for all nonempty, compact subsets L of ${\displaystyle \mathbb {X} }$.

Informally, If ${\displaystyle L}$ is close to being stabilized by the IFS, then ${\displaystyle L}$ is also close to being the attractor of the IFS.

• Michael Barnsley
• Barnsley fern

• Barnsley, Michael. (1988). Fractals Everywhere. Academic Press, Inc. ISBN 0-12-079062-9.

• A description of the collage theorem and interactive Java applet at cut-the-knot.
• Notes on designing IFSs to approximate real images.
• Expository Paper on Fractals and Collage theorem