2.1 Coupled Dynamics and Complex Systems
3.1.3 The Symbolic Formalism
There exist several notations and conventions that apply to rank orders and their associated permutations. Since the conventions of different fields may deviate from each other, it is of importance that the appearance of the equations does not depend on the viewpoint (e. g. if a variable is interpreted as a rank order or as a permutation). In this section, the formalism used in this work is presented and compared to other notations from related fields. Since not all common conventions are compatible, non-intuitive notations can not always be avoided.
Symbols
Definition 3.4. The group of permutations ofn elements is defined as the set of functions
Sn:={a:A→A|abijective}, where A:={1, . . . , n}. (3.4)
The relation between these functions and the exchange of objects can easily lead to mis- understandings, hence it is important to first make clear the terms used in this context:
Definition 3.5 (Items and positions). Let a∈Sn be a permutation. Then, a(i) =j is under- stood as
’The new item on the position i is the one from position j.’
Fig. 3.3: Items and positions. a(i) =
j means thatitem number j is put on
positionnumberi. Note that the arrows point froma(i) toi, which is a possible source of confusion (and a consequence of the mathematical standard notation for permutations).
Elements ofSn can be referenced in several ways (specific expressions are given forn= 4, wherever considered helpful). Let a∈Sn be a permutation.
a) The most straightforward way to write down a function is to state its value for every possible argument, in the so-calledtwo-line notation:
1 2 . . . n
a(1) a(2) . . . a(n)
. (3.5)
Since the first line is canonical, it is enough to write down the second one,
[a(1), . . . , a(n)], (3.6)
which is called one-line notation and in this work is always denoted in square brackets, in order to avoid ambiguities.
b) A further important convention is cycle notation: as mentioned, any permutation can be written as a product of disjunctcycles of the form
(x, a(x), a2(x), . . . , ak−1(x)), (3.7)
where k is the minimal positive integer for which ak(x) =x, and those cycles are unique (up to sequential rearrangement). Note that, for the given notation, a cycle (1,2,3,4) shifts the entries of a 4-vector to the left, i. e. the new entry on position 1 is the one from position 2, etc. For both one-line and cycle notation, I will omit commas, as long as the expressions are still unambiguous, e. g., for n <10 it is clear that each digit stands for a number. c) For visualizing a symbol, I use diagrams ofa(x) versusx (fig. 3.4a), which I refer to as an
order plot. Definition 3.5 implies that the rank order of a delay vector is associated to the permutation that generates this rank order when applied to the columns of an ascending rank order. This implies that an exemplary delay vector with entries 1,2,4,3 corresponds to a state symbol with one-line notation [1243]. As a result, the one-line notation (and the order plot) of a state symbol is a discretization of the corresponding delay vector.
d) Permutations can be written as permutation matrices. The matrix A ∈ Rn×n that corre-
sponds to a permutationa∈Sn is defined by Aij :=
(
1 if j =a(i)
0 else. (3.8)
Note that, by convention, the rows of a matrix are counted from top to bottom, while the ordinate of an order plot is oriented from bottom to top, which is a potential source of confusion (compare fig. 3.4a and b).
Fig. 3.4: a) An order plot can be understood as a function graph (a(i) versusi). b) The correspond- ing permutation matrix (note that the rows are counted from top to bottom).
1 0 0 0
0 1 0 0
0 0 0 1
0 0 1 0
Composition of SymbolsThe group of permutations is not commutative. Therefore, it is important to bring together the different conventions about left and right actions. The standard way to define the composition of permutations is the convention for the composition of functions:
Fig. 3.5: Compositionabof two order plots
a= [1342] andb= [2314].
This formulation comes from the original definition 3.4. In principle, this is enough to make all symbol equations of this work well-defined. However, for reasons of clarity, I will briefly explain how the composition looks like in terms of the other notations. As a numeric example, I consider the symbols a := (234) = [1342] and b := (123) = [2314], and their composition
ab= (13)(24) = [3412].
a) In the cycle notation, consider the composition example (234)(123), evaluated on the argu- ment 2. The right cycle acts first, mapping 2 to 3, which is then mapped to the final result 4 by the left cycle (similarly for all other arguments). This corresponds to the formulation
(234)(123) = (13)(24).
b) Fig. 3.5 illustrates the composition of two order plots, placing item a(b(i)) at position i. Note that the arrows point from ab(i) to i(same as in fig. 3.3). This perspective translates as
[1342][2314] = [3412].
c) Now consider the action of a permutation on an order plot. From the perspective of an order plota, the action of bpermutes the output ofa(see also fig. 3.5): itema(i) now goes to position b−1(i) (instead of position i). This corresponds tob permuting the columns of
a, or more exact:
’the new column b−1(i) shall be the one that was column ibefore’,
which is equivalent to
’the new column i shall be the former column b(i)’. This perspective corresponds to the formula
Fig. 3.6:Example (n= 4). The compositionabof symbolsa:= (234) = [1342] andb:= (123) = [2314] can be interpreted as a left-action ofaonb (left side), or as a right action ofb ona(right side). Both lead to the same symbolab= [3412] (middle).
Conversely, from the perspective of an order plotb,aexchanges the items before they enter
b (sending item a(i) instead of item i), corresponding toa−1 permuting of the rows of b. This formulation translates as
(234)[2314] = [3412].
In summary (see also fig. 3.6),
ab= (
bpermuting the columns of a(right action of bon a)
a−1 permuting on the rows ofb (left action ofa onb). (3.10)
In this work, the term ’apply a permutation to a rank order’ refers to the right action, if not explicitly specified otherwise.
d) It is straightforward to check, that the multiplication of matrices works analogous to the reasoning for order plots, but we should take a look at how row and column vectors are included in the formalism of matrix multiplication. One-line notations can be interpreted as row vectors:
[a(1), . . . , a(n)] = [1, . . . , n]A= 1A, (3.11) with aand A as above. Note that row vectors are written in square brackets, in order to prevent ambiguities with the cycle notation. For the column vector, this implies
A=A1 =A[1, . . . , n]T = ([1, . . . , n]AT)T. (3.12) Hence, the column vector that corresponds to the matrix representationA(and the symbol
a= [a(1), . . . , a(n)]) is
[a−1(1), . . . , a−1(n)]T. (3.13)
e) In programming, symbols are usually composed via index notation, where (examples for
a, bas above)
a[b] :=ab= [1342][2314] (3.14) is translated as’in the array[1,3,4,2], take the entries at the position2,3,1 and4’. In this formulation, the index notation is compatible with the rest of the formalism discussed here.
Transfer Symbols
A transfer symbol was introduced as the permutation that connects two rank orders. Formally, this can be understood as a left or right action. For two symbolsaand b, the transfer symbol
t, as introduced by Monetti et al. [65], refers to the right action:at=b, which is equivalent to
t=a−1b. (3.15)
This is the permutation that generates b, when applied to the columns of a (see fig. 3.2). Alternatively, a left-action transfer symbolt0can be defined byt0a=b, which leads tot0=ba−1. When not explicitly stated otherwise, the term transfer symbol refers to the right action.
Concerning the relation between right- and left-transfer symbols, consider two pairs of symbols with the same (right-) transfer symbol,
a−1b=A−1B. (3.16)
Then, the symbols differ by a common factor
Aa−1 =Bb−1 =:c. (3.17) The left-transfer symbols are ba−1 and
BA−1 =cba−1c−1. (3.18) Thus, one version of the transfer symbol determines the conjugacy class (see definition 3.7 in section 3.2.1) of the other. Setting c:=aleads to
t=a−1b=a−1 (ba−1)a=a−1 t0a. (3.19) Therefore, both versions of a transfer symbol share the same conjugacy class. Furthermore, for fixed t, any amay appear, hencet0 can be any element from this class.