Definition of the Subject and Its Importance
Cellular automata in hyperbolic spaces, in short hyperbolic cellular automata (abbreviated HCA), consists in the implementation of cellular automata in the environment of a regular tiling of a hyperbolic space.
The first implementation of such an object appeared in a paper by the present author and Kenichi Morita in 1999 (see Margenstern and Morita 1999). In this first paper, a first solution to the location problem in this context was given. The paper also focused on one advantage of this implementation: it allows to solve NP problems in polynomial time. In 2000, a second paper appeared, by the present author, where a decisive solution of the location problem was given.
The study of HCAs is a new domain in computer science, at the border with mathematics and physics. They involve hyperbolic geometry as well as elementary arithmetic and algebra with the connections of polynomials with matrices and also some theory of fields. They also...
Abbreviations
- Dodecagrid:
-
The tiling {5, 3, 4}. This tessellation lives in the hyperbolic 3D space. Its basic polyhedron is the dodecahedron constructed on regular rectangular pentagons.
- Fibonacci sequence:
-
Sequence of natural integers, denoted by f n and defined by the recurrent equation \( {f}_{n+2}={f}_{n+1}+{f}_n, \) for all n ∈ ℕ and by the initial values f 0 = f 1 = 1.
- Heptagrid:
-
The tiling {7, 3}, necessarily in the hyperbolic plane. Seven sides and three tiles around a vertex. It is called ternary heptagrid in several papers by the author and its coauthors also in Margenstern (2007c, 2008b).
- Hyperbolic geometry:
-
Geometry, discovered by Nikolaj Lobachevsky and Jànos Bolyai, independently of each other and around 1830. This geometry satisfies the axioms of Euclidean geometry, the axiom of parallels being excepted and replaced by the following one: through a point out of a line, there are exactly two parallels to the line. In this geometry, there are also lines which never meet: they are called non-secant. They are characterized by the existence, for any couple of such lines, of a unique common perpendicular. Also, in this geometry, the sum of the interior angles of a triangle is always less than π. The difference to π defines the area of the triangle. In hyperbolic geometry, distances are absolute: there is no notion of similarity. See also Poincaré’s disk.
- Invariant group of a tiling:
-
Group of transformations which defines a bijection on the set of tiles. Usually, in a geometrical space, they are required to belong to the group of isometries of the space.
- Pentagrid:
-
The tiling {5, 4}, necessarily in the hyperbolic plane. Five sides and four tiles around a vertex. The angles are right angles.
- Poincaré’s disk:
-
Model of the hyperbolic plane inside the Euclidean plane. The points are those which are interior to a fixed disk D. The lines are the trace in D of diameters or circles which are orthogonal to the border of D. The model was first found by Beltrami and then by Poincaré who also devised the half-plane model also called after his name. The half-plane model is a conformal image of the disk model.
- Tessellation:
-
Particular case of a finitely generated tiling. It is defined by a polygon and by its reflections in its sides and, recursively, of the images in their sides.
- Tiling:
-
Partition of a geometrical space; the closure of the elements of the partition is called the tiles. An important case is constituted by finitely generated tilings: there is a finite set of tiles G such that any tile is a copy of an element of G.
- Tiling {p, q}:
-
Tessellation based on the regular polygon with p sides and with vertex angle \( \frac{2\uppi}{q}. \)
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Acknowledgment
The author again thanks Andrew Adamatzky for giving him the task to write the first issue of this entry. He is also much in debt to Andrew Spencer for asking him this new version.
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Margenstern, M. (2015). Cellular Automata in Hyperbolic Spaces. In: Meyers, R. (eds) Encyclopedia of Complexity and Systems Science. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-642-27737-5_53-5
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