The Generalized Weave: Topological Covers and the State-Space of Substitution Tilings.
Abstract
We introduce a universal operator, the Generalized Weave ($W$), which acts upon any edge-to-edge aperiodic substitution rule to generate an infinite family of topologically distinct tilings. By cloning prototiles and permuting child-parent relationships via a winding vector, we demonstrate that a single substitution rule defines a multidimensional state-space of non-MLD covers. This result suggests that the current catalog of aperiodic tilings represents only the origin points of vast, unexplored topological manifolds.
Background
Substitution rules are important in the study of aperiodic tilings since they provide a compact method to rigorously define a tiling, and to generate the tiling. There is some variation in the way that substitution rules are defined, with edge-to-edge rules being the most well-behaved and standard. Often, if a rule exists which is not edge-to-edge, it may be modified, either by removing some tiles from the prototiles, or by cutting tiles that appear on the edge, so that the resulting tiling is edge-to-edge.
In the study of aperiodic tilings, mutual local derivability (mld) is used as a classifier. If two tilings are mld they are said to belong to the same mld class. If a tiling is discovered that is non-mld with existing known tilings, then it is thought of as a novel aperiodic tiling of the plane.
A tiling $A$ is locally derivable from tiling $B$ if given a patch of the tiling of finite radius $r$, the tile that appears at the same place in $B$ can be derived from the tile that appears there in $A$. And vice versa.
While a substitution rule generates one tiling, it is shown below that there exists an operator $W$ (informally called the Generalized Weave), that generates an infinite set of substitution rules from any given rule, and thus creates an infinite set of tilings which, after a process of deduplication, belong to distinct MLD classes.
The weave operates by cloning the prototiles within the rule, and weaving together the prototile-tile relationships. It is called the generalized weave because it operates on any edge-to-edge substitution rule.
Theory
We adopt the following notation:
$W(S, E, C, V)$
where $W$ is the Generalized Weave Operator, which generates a set of substitution rules, and hence tilings, given the following. $S$, the original substitution rule, $E$, the expansion factor, $C$, the cloning number, and $V$, the winding vector, where $E$ and $C$ are natural numbers and $V$ is a mapping $V: {1, \dots, M} \to {0, \dots, C-1}$, where $M$ is the number of child tiles in the expanded substitution $S^E$
$W$ is defined as follows: Beginning with $S$, $S$ is applied to itself $E$ times, meaning that if the original inflation factor was $\lambda$ the new inflation factor is $\lambda^S$.
To preserve topological consistency under the operator $W$, the prototiles are endowed with an orientation vector, ensuring that the weaving is applied relative to the tile’s local coordinate system.
The tiles within the prototiles are marked with a number - by convention from top to bottom and then from left to right.
The prototiles within $S$ are then cloned $C$ times, marking the prototiles with new colors. We say that the prototiles form a matrix, with the original prototiles occupying the first column, and each succesive clone of prototiles occupying a subsequent column.
Having cloned, and assigned colors, all relationships between prototiles and tiles are within a column.
The winding numbers are applied as follows: each tile numbered $i$ in column $j$ is replaced by the corresponding tile in column $(j + V[i]) \pmod C$.
Theorem
Given the operator $W(S, E, C, V)$, the resulting set of substitution rules, when culled, generates tilings which are non-MLD with $S$, and also pairwise non-MLD.
Culling means to remove duplicates which may arise for the following two reasons:
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The tiling graph may collapse into a disjoint graph, meaning that the tiling is not woven, and hence will be MLD with a lesser value of $C$.
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The tiling may be isomorphic with another tiling.
Proof
First we prove, by construction, that the tilings exist.
The method described above, when applied to a substitution rule with edge-to-edge matching rules, generates an infinite set of substitution rules which are as valid as the original. The fact that they are edge-to-edge means that all edges within a row of the matrix have identical geometry, even though they may be composed of different tiles.
And now consider the non-MLD property of the tilings.
Because the substitution is aperiodic, the “path” from a tile back to the common ancestor of a neighbor can be arbitrarily long. If the winding vector $V$ creates a “non-trivial cycle” through the clones, then a local observer at radius $r$ cannot know the “phase” of the weave without knowing the “index” of the tile in the infinite hierarchy.
Since the hierarchy is infinite, the information required is non-local. This is the definition of Non-MLD.
For any finite radius $r$, there exists a generation $G$ such that the local patch is contained within a single super-tile. However, the ‘phase’ of the weave at that patch is determined by the sequence of choices made at every generation $g > G$. Since an aperiodic substitution possesses no global period, this ‘ancestral path’ cannot be determined by local observation, proving $S_2$ is not locally derivable from $S_1$.
Examples
We consider some examples, displaying patches of the resulting tilings. As examples we use Penrose P3 and Ammann-Beenker A5.
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A simple example: $W(P3, 1, 2, [1,0,0,0,0] )$
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$W(P3, 2, 6, [2, 3, 0, 0, 0])$
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$W(A5, 1, 2, [1, 0, 0, 0, 0,0,0,0,0,0,0,0])$
Conclusion
While we have shown the existence of an infinite companion set of tilings for any edge-to-edge substitution rule, it follows from Goodman-Strauss[ref] that a corresponding tileset exists.