Graph of a dynamical system
In mathematics, a Bratteli diagram is a combinatorial structure: a graph composed of vertices labelled by positive integers ("level") and unoriented edges between vertices having levels differing by one. The notion was introduced by Ola Bratteli in 1972 in the theory of operator algebras to describe directed sequences of finite-dimensional algebras: it played an important role in Elliott's classification of AF-algebras and the theory of subfactors. Subsequently Anatoly Vershik associated dynamical systems with infinite paths in such graphs.
Definition.
A Bratteli diagram is given by the following objects:
A customary way to pictorially represent Bratteli diagrams is to align the vertices according to their levels, and put the number "b""v" beside the vertex "v", or use that number in place of "v", as in
An ordered Bratteli diagram is a Bratteli diagram together with a partial order on "E""n" such that for any "v" ∈ "V""n" the set { "e" ∈ "E""n"−1 : "r"("e") = "v" } is totally ordered. Edges that do not share a common range vertex are incomparable. This partial order allows us to define the set of all maximal edges "E"max and the set of all minimal edges "E"min. A Bratteli diagram with a unique infinitely long path in "E"max and "E"min is called "essentially simple".
Sequence of finite-dimensional algebras.
Any semisimple algebra over the complex numbers C of finite dimension can be expressed as a direct sum ⊕k Mnk(C) of matrix algebras, and the C-algebra homomorphisms between two such algebras up to inner automorphisms on both sides are completely determined by the multiplicity number between 'matrix algebra' components. Thus, an injective homomorphism of ⊕"k"=1"i" "M""n""k"(C) into ⊕"l"=1"j" "M""m""l"(C) may be represented by a collection of positive numbers "a""k", "l" satisfying Σ "n""k" "a""k", l ≤ "m""l". (The equality holds if and only if the homomorphism is unital; we can allow non-injective homomorphisms by allowing some "a""k","l" to be zero.) This can be illustrated as a bipartite graph having the vertices marked by numbers ("n""k")"k" on one hand and the ones marked by ("m""l")"l" on the other hand, and having "a""k", "l" edges between the vertex "n""k" and the vertex "m""l".
Thus, when we have a sequence of finite-dimensional semisimple algebras "A""n" and injective homomorphisms "φ""n" : "A""n"' → "A""n"+1: between them, we obtain a Bratteli diagram by putting
 "V""n" = the set of simple components of "A""n"
(each isomorphic to a matrix algebra), marked by the size of matrices.
 ("E""n", "r", "s"): the number of the edges between "M""n""k"(C) ⊂ "A""n" and "M""m""l"(C) ⊂ "A""n"+1 is equal to the multiplicity of "M""n""k"(C) into "M""m""l"(C) under "φ""n".
Sequence of split semisimple algebras.
Any semisimple algebra (possibly of infinite dimension) is one whose modules are completely reducible, i.e. they decompose into the direct sum of simple modules. Let formula_1 be a chain of split semisimple algebras, and let formula_2 be the indexing set for the irreducible representations of formula_3. Denote by formula_4 the irreducible module indexed by formula_5. Because of the inclusion formula_6, any formula_7-module formula_8 restricts to a formula_3-module. Let formula_10 denote the decomposition numbers
 formula_11
The Bratteli diagram for the chain formula_1 is obtained by placing one vertex for every element of formula_2 on level formula_14 and connecting a vertex formula_15 on level formula_14 to a vertex formula_17 on level formula_18 with formula_19 edges.
Examples.
(1) If formula_20, the ith symmetric group, the corresponding Bratteli diagram is the same as Young's lattice.
(2) If formula_3 is the Brauer algebra or the Birman–Wenzl algebra on "i" strands, then the resulting Bratteli diagram has partitions of "i"–2"k" (for formula_22) with one edge between partitions on adjacent levels if one can be obtained from the other by adding or subtracting 1 from a single part.
(3) If formula_3 is the Temperley–Lieb algebra on "i" strands, the resulting Bratteli has integers "i"–2"k" (for formula_22) with one edge between integers on adjacent levels if one can be obtained from the other by adding or subtracting 1.
References.
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