Review Article | Open Access
Juan A. Aledo, Silvia Martinez, Jose C. Valverde, "Parallel Dynamical Systems over Graphs and Related Topics: A Survey", Journal of Applied Mathematics, vol. 2015, Article ID 594294, 14 pages, 2015. https://doi.org/10.1155/2015/594294
Parallel Dynamical Systems over Graphs and Related Topics: A Survey
In discrete processes, as computational or genetic ones, there are many entities and each entity has a state at a given time. The update of states of the entities constitutes an evolution in time of the system, that is, a discrete dynamical system. The relations among entities are usually represented by a graph. The update of the states is determined by the relations of the entities and some local functions which together constitute (global) evolution operator of the dynamical system. If the states of the entities are updated in a synchronous manner, the system is called a parallel dynamical system. This paper is devoted to review the main results on the dynamical behavior of parallel dynamical systems over graphs which constitute a generic tool for modeling discrete processes.
Modeling discrete processes is one of the most important tasks in modern mathematics. In fact, several mathematical concepts have become fundamental in order to establish discrete mathematical models for several phenomena coming from science and engineering. Some of them are Boolean algebras and functions [1, 2], together with other topics as graphs [1, 2] and discrete dynamical systems [3–6].
Boolean algebras give symbolic form to Aristotle’s system of logic. In the second half of the nineteenth century, the English mathematician George Boole (1815–1864) defined an algebraic structure which encodes several rules of relationship between mathematical quantities limited to two possible values: true or false, mathematically modeled by 1 or 0. This algebraic structure is currently known as Boolean algebra. The results of the study were published in a survey  entitled “An Investigation of the Laws of Thought, on Which Are Founded the Mathematical Theories of Logic and Probabilities” in 1854. Nevertheless, after almost a century, Claude E. Shannon was the first one who established how Boolean algebras could be applied to on-off circuits, where the presence of signal is characterized by 1 and the absence by 0. In the forties, his master’s thesis  entitled “A Symbolic Analysis of Relay and Switching Circuits” made Boole’s theoretical work become a fundamental mathematical tool for designing and analyzing digital circuits.
Graphs appear in many different fields of science and engineering as a tool to represent the relations among the elements of a system [9–15]. They constitute a powerful tool to perceive more clearly problems and to face up to them, especially when there is a finite number of elements to deal with. This occurs in discrete processes and so they are basic for the mathematical formalization of many of these processes.
The notion of dynamical system is the mathematical formalization of the general scientific concept of deterministic process [5, 6]. It involves a set of possible states of the process, which is named state space or phase space, and a law of the evolution or evolution operator in time. Thus, this notion is fundamental in order to model any phenomenon whose future asymptotic state needs to be known.
These three concepts are mixed properly to construct a mathematical model named parallel dynamical system (PDS) that allows us to formalize and analyze the dynamical behavior of discrete processes.
This mathematical model constitutes a generalization of other relevant ones which appeared previously in the literature, as cellular automata (CA) [16–25] or Boolean networks (BN) [14, 26–29]. (The abbreviations PDS, SDS, CA, and BN will be written for the singular and plural forms of the corresponding terms, since it seems better from an aesthetic point of view).
A CA has been traditionally conceived as a grid of cells, where each cell has a state belonging to a finite set (usually ), such that all the cells evolve synchronously in discrete time steps. The updating of the state of every cell is realized according to a common local function affecting only the neighbors of such a cell. Thus, if is the state value of a cell at the time , the cell value is updating by applying a local function on the cells in the neighborhood of the cell .
CA has revealed as a suitable mathematical model to capture the essential features of digital computers: synchronicity, regular distribution, and locality of iterations. Although they were introduced for the first time in the works of Ulam and von Neumann , they became of public interest in 1970, when Martin Gardner published  with an explanation of John Conway’s “Game of Life” in Scientific American (see also ).
In , Wolfram analyzed a set of CA and showed that, despite their simple construction, some of them are capable of a complex behavior. Later, in , based on a deeper research, he suggested that many one-dimensional cellular automata fall into four basic behavior classes: three of them exhibiting a similar behavior to fixed points, periodic orbits, and chaotic attractors, and the fourth one such that its asymptotic properties are undecidable. Wolfram published other important papers which are compiled in .
A BN of size and connectivity consists of interconnected vertices, each one having inputs. The update of the state of any vertex is determined by the (directed) dependency relations and local rules which are given by Boolean functions. Hence, BN are a generalization of (finite) Boolean CA.
BN appeared, almost at the same time, in applied models created for the simulation of aspects of the behavior of biological systems. This occurred in , where Kauffman constructed molecular automata for modeling a gene as a binary (on-off) device and studied the behavior of large, randomly constructed nets of these binary genes (see also ). For this reason, BN are also known as Kauffman (net) models. The results by Kauffman suggest that if each gene is directly affected by two or three genes then the system behaves with great order and stability and presents cycles.
In the last two decades, many works have focused their attention on mathematical modeling of several computer processes. The first one of a series of these works was , which constituted an important step in the development of mathematical foundations for the theory of computation. In this work, sequentially updated cellular automata (SCA) over arbitrary graphs are employed as a paradigmatic framework. This first work was followed by [34–36], where the authors developed this theory, analyzing the asymptotic behavior of such mathematical models. The formal definition of PDS appeared for the first time in  and it constitutes a fundamental issue for the posterior development of the results in this research line.
Computer processes involve generation of dynamics by iterating local mappings. In fact, a computer simulation is a method for the composition of iterated mappings, typically on local dependency regions . It means that the mappings have to be updated in a specific manner, that is, an update schedule. Update scheduling is also a commonly studied aspect in discrete event simulations [37–39]. Mathematical modeling of computer processes has resulted very useful in order to predict what can occur after executing these processes.
In the computational context, it is common to rename the local mappings as entities (cells in the language of cellular automata theory), which are the lowest level of aggregation of the system. In computer processes, there are many entities and each entity has a state at a given time (see [33–35]). The update of states of the entities constitutes an evolution in time of the system, that is, a discrete dynamical system (see [9, 40]).
In this sense, each entity of the system could be activated or deactivated. Thus, it is natural to consider that its state . The evolution or update of the system is implemented by local functions which are restrictions of a global function or together constitute this global one. That is, for updating the state of any entity, the corresponding local function acts only on the state of that entity itself and the states of the entities related to it. The relations among entities are usually represented by a graph which is called the dependency graph of the system.
Thus, the update of the states is determined by the dependency graph relations of the entities and the local functions which together constitute the (global) evolution operator of the dynamical system. If the states of the entities are updated in a parallel (or synchronous) manner, the system is called a parallel dynamical system [9, 41–43], while if they are updated in a sequential (or asynchronous) order, the system is named sequential dynamical system (SDS) [9, 33–36, 40, 44–46].
CA, when finite, can be considered as a special kind of PDS by considering cells as entities. On the other hand, BN are a generalization of (finite) Boolean CA but, at the same time, a particular case of PDS by considering nodes as entities. One of the main differences with CA is that, in BN, the state of each node is not affected necessarily by its neighbors but potentially by whichever node in the network. Thus, the uniform structure of neighborhood in CA disappears. However, some homogeneity remains, since each node is affected by connections with other (or the same) entities. This homogeneity makes BN a particular case of PDS, since in PDS connections can be totally arbitrary.
Since CA and BN are special cases of PDS, their applications can be assumed as applications of PDS. Some applications of CA and BN that appear in the literature are enumerated below in order to give an idea of the great interest of this tool for modeling discrete processes from science and engineering.
As said before, Boolean networks appear for the first time in , where Kauffman constructed molecular automata for modeling a gene as a binary (on-off) device (see also ). Up to date, these results continue being developed as can be seen in , where they consider asynchronous stochastic update, , where probability is introduced in the directed dependency graph which they call random Boolean network, or , where a Kauffman net model shows that eukaryotic cells are dynamically ordered or critical but not chaotic. Since Kauffman nets can be seen as particular cases of directed dependency graphs that are considered here, the results shown in this review article could be useful in the development of some aspects of biological systems and biochemical control networks.
In the eighties, two important works appeared relating CA and ecological modeling [18, 49]. In fact,  presents CA as a paradigm for ecological modeling. Later, in  CA were used to simplify spatial complexity in the geometry of ecological interactions. Finally, in  the authors studied the links between CA and population dynamics.
On the other hand, in  a parallel discrete dynamical model with three evolution local functions is used in order to analyze the structure of a partially ordered set (poset) of signed integer partitions whose main properties are actually not known. The authors affirm that this model is related to the study of some extremal combinatorial sum problems. Besides, in , a class of lattices and Boolean functions are employed to study the truth of a mathematical conjecture. Other related interesting works in this research line are [54–57].
Also in sociology, there exist some works where CA are used for modeling the corresponding systems (see, for instance, [60, 61]). Besides, in the literature one can find other works where this kind of systems are employed for other applications as the simulation of two-lane traffic  or the generation of rhythmic structure in music .
According to , one of the main goals in the study of a dynamical system is to give a complete characterization of its orbit structure. Actually, this is the main purpose of the review paper in relation with parallel dynamical systems over graphs: to show as much information about the orbit structure as possible, based on the properties of the dependency graph and the local Boolean functions which constitute the evolution law of the system.
In this particular case, as the state space of the system is finite, every orbit is periodic or eventually periodic. Note that, for a system with entities, the number of possible states is equal to . Thus, for example, if , then the number of possible (initial) states of the system is greater than a million. Therefore, computational studies become inefficient when is big enough and can only analyze small systems or a low number of initial states.
In view of that, the unique way to give general results for these systems is to find out their properties analytically, as done in [41–43, 64, 65] in order to describe the different coexistent periodic orbits of PDS. Following Derrida and Pomeau , the most interesting questions about these systems concerning their orbit structures are the following.(1)What is the length of the limit cycles?(2)What is the number of different limit cycles?(3)If one considers two different initial states, when do they arrive in the same limit cycle?
Actually, these are the main questions reviewed in this paper for PDS. More specifically, this paper reviews the full characterization of the orbit structure of PDS over undirected graphs covering all the casuistry, since it shows the behavior of any parallel dynamical system regardless the type of graph, the number of entities, and the relationships among them. Also, essential properties of the orbit structure of PDS over directed dependency graphs are revised. Besides, two extensions of the concept of PDS are shown.
The paper is organized as follows. In the next section we present theoretical foundations of PDS and compare them with CA and BN in detail. In Section 3, the most important results on the dynamics of PDS are reviewed. Section 4 is devoted to reviewing two extensions of this mathematical model. A linearization algorithm for the computation of orbits in PDS is addressed in Section 5. The paper finishes by setting out several interesting future research directions for the development of these kinds of models.
2. Theoretical Foundations of PDS and Comparative Study with Related Topics
2.1. Theoretical Foundations
2.1.1. Dependency Graphs
In computer processes, there are many entities and each entity has a state at a given time (see [33–36]). Entities are related and they get information from the entities in their neighborhood. Usually, in order to get a graphical idea of the situation, every entity is represented by a vertex of an undirected graph and two vertices are adjacent if their states influence each other in the update of the system. The undirected graph so built is called the (undirected) dependency graph of the system (see ).
If we denominate this graph , where is the vertex set and is the edge set, then, for each vertex/entity , , it is natural to consider that its state . That is, the entity can be activated or deactivated.
On the other hand, for every vertex/entity and every subset , we will consider all the vertices that interfere with them. Thus, we denote bythe set of vertices that are adjacent to the vertex .
Nevertheless, in many occasions, the process of information exchange is not bidirectional . This situation can be represented by an arc whose initial vertex is the influencing entity and the final vertex corresponds to the influenced entity, obtaining a directed graph or digraph of relations. The directed graph so built will be called the directed dependency graph of the system. In order to unify the notation, it will be also denoted by , although in this case is a set of arcs instead of edges. With the same aim, given a directed dependency graph and , will stand for the set of vertices such that there exists an arc from to .
Recalling that, given vertices , the distance between them is defined as the length of the shortest path from to , and the diameter of the digraph as
2.1.2. Boolean Algebra and Boolean Functions
As the natural way to consider if the state of any entity is activated or deactivated, the basic Boolean algebra will play a fundamental role for the theoretical foundations of the model. Moreover, the (general) structure of Boolean algebra is essential to naturally capture the situation where entities are composed by some subentities, such that each of them can be activated or deactivated.
Mathematically, a Boolean algebra can be defined  as an algebraic structure where is a set, are two inner operations defined on , is an application from to , and are two distinguished elements of such that they satisfy certain laws (idempotence, commutative, associative, absorption, distributive, existence of neutral and universal elements, and existence of complement element).
It is common to denote the operation by + and to call it addition operation of the Boolean algebra. Likewise, the operation is usually denoted by and called multiplication operation of the Boolean algebra.
It is natural to introduce the algebraic structure of Boolean algebra as a Boolean Lattice which is bounded, distributive, and complemented . In fact, in  a definition where only commutative, idempotence, distributive, and complement laws are required is given, which fits better the original definition by George Boole.
If is any Boolean algebra, then is also a Boolean algebra, where the operations are performed coordinate by coordinate; that is,(i),(ii),(iii).
In this Boolean algebra, the neutral and universal elements are given, respectively, by and .
Two Boolean algebras and are isomorphic (as Boolean algebras)  if there is a bijection such that, for all , it satisfies(i),(ii),(iii).
In particular, we have that and .
One of the most important facts in the algebraic structure of Boolean algebra is the existence of some basic elements, named atoms of the Boolean algebra, which allow us to control the rest of the elements of the structured and focus on them the essence of the structure. An atom is every element , different from the neutral one , which verifies that if and only if or .
The atoms allow us to express any element of the Boolean algebra, different from the neutral one , as a disjunction of them in a unique way (up to the order). Moreover, the well-known Stone’s theorem (see ) asserts that all the Boolean algebras with the same number of atoms are isomorphic. In particular, every Boolean algebra with atoms is isomorphic to the Cartesian product of copies of the basic algebra , what is essential for obtaining more general results on this model . As a corollary, if a Boolean algebra has atoms, then has elements.
Another important feature is the duality principle in Boolean algebras, which means that if a statement is the consequence of the definition of Boolean algebra, then the dual statement is also true. The dual of a statement in a Boolean algebra is the statement obtained by interchanging the operations and and the elements and in the original statement .
Concerning Boolean algebras, another important concept throughout this work is the following.
Definition 1. Let be a Boolean algebra and . A Boolean function of variables is a function of the formwhere is obtained from using the inner operations defined on , the application from to , and the elements .
A Boolean function describes how to determine a Boolean output from some Boolean inputs. Thus, such functions play a fundamental role in questions as design of circuits or computer processes . In our context, they correspond to components of the evolution operator of the dynamical system.
An important point is that the set of all the Boolean functions of variables is also a Boolean algebra (see ).
Maxterms and minterms are both special cases of Boolean functions.
Definition 2. A Boolean function of variables, , that only uses the disjunction operator , where each of the variables appears once in either its direct or its complemented form, is called a maxterm.
In this sense, the simplest maxterm corresponds to the one where each of the variables appears once in its direct form; that is,
With all the variables in their complemented form, we have the maxterm NAND:
Minterm is the dual concept of maxterm, changing the disjunction operator for the conjunction one.
Definition 3. A Boolean function of variables that only uses the conjunction operator , where each of the variables appears once in either its direct or its complemented form, is called a minterm.
The simplest minterm corresponds to the one where each of the variables appears once in its direct form; that is,
With all the variables in their complemented form, we have the minterm NOR:
As one can check, there exist exactly maxterms of variables, since a variable in a maxterm expression can be either in its direct or in its complemented form, and dually minterms. Since the minterms are the atoms of the Boolean algebra which is constituted by all the Boolean functions of variables, there are exactly Boolean functions of variables.
In particular, see [2, 69], any Boolean function, except (resp., ), can be expressed in a canonical form as a conjunction (resp., disjunction) of maxterms (resp., minterms). Therefore, it is natural to begin the study of the dynamics with these basic Boolean functions.
2.1.3. Dynamical Systems
Mathematically, a dynamical system is defined as a triple where is the state space, is the time set, and is the evolution operator, verifying the following.(1) is a set.(2) is a number set.(3) is a map satisfyingThe set is often a metric space in order to determine the distances between two different states, and it is also called phase space, due to classical mechanics.
The most general way in which the evolution operator is given is by means of a family of maps depending on :which transforms any initial state into some state at time ; that is .
Depending on the time set , it is distinguished between discrete and continuous dynamical systems:
In this review, we deal with a special kind of discrete dynamical systems. A discrete dynamical system is fully specified by defining only one map , named the time-one map of the system, since for any other ,
In our particular case, as the state space is finite, the time-one map can be represented by a table which is called lookup table for the state updating.
When the evolution operator is defined for both negative and positive values of the time, the system is called invertible. In such a system the initial state defines not only the future states, but its past behavior as well. In our context, the invertibility of the system appears associated to the reversibility problem [70, 71].
We actually determine a dynamical system over a (directed or undirected) dependency graph by associating to each vertex a state and a local map defined on the states of the vertices in and which returns its new state .
In fact, the evolution or update of a system over a (directed or undirected) dependency graph is implemented by local (Boolean) functions. These local functions are either restrictions of a global function or jointly constitute the (global) evolution operator of the system.
If the states of the entities are updated in a parallel manner, the system is called a parallel dynamical system (PDS) [9, 41, 42], while if they are updated in a sequential order, the system is named a sequential dynamical system (SDS) (see [9, 40, 46]). More precisely.
Definition 4. Let be a (directed or undirected) graph with . Then a mapwhere is the updated state of the entity/vertex by applying locally the function over the states of the vertices in , constitutes a discrete dynamical system called a parallel (discrete) dynamical system (PDS) over with evolution operator , which will be denoted by or when specifying the dependency graph is not necessary.
Definition 5. Let be a (directed or undirected) graph with and a permutation on . Then a mapwhere is the update function on the state vector which updates the state of the vertex while keeping the other states unchanged, constitutes a discrete dynamical system called a sequential (discrete) dynamical system (SDS) over with evolution operator , which will be denoted by or when specifying the dependency graph is not necessary.
Basic objects associated to a dynamical system are its orbits and consequently the phase portrait, that is, the partitioning of the state space into its orbits.
Definition 6. The orbit of a dynamical system starting at is the ordered subset of the state space given by
Orbits of a discrete dynamical system are ordered sequences of states in the phase space that can be enumerated by increasing integers.
It is worthwhile to recall that, given a discrete dynamical system with global evolution function and an initial state , the following terminology is used.(i) is called the successor of the state . Observe that, since a dynamical system is deterministic, there exists exactly one successor for each initial state, but this initial state could be the successor of more than one state of the system that are called its predecessors. When this occurs the system is often called a dissipative system and the in-degree of the initial state is the number of its predecessors (out-degree is 1 for every initial state).(ii)If , then is called a fixed point of or equilibrium state of the system. The notation represents the set of all the fixed points of the dynamical system with global evolution function .(iii)If there exists an integer , such that and for any integer , , then is called a periodic point of or a periodic state of the system and is called the period of the orbit of . The notation is adopted to denote the set of all the periodic points of . Usually, a periodic orbit is called a cycle, but we do not use this name in order to avoid confusions with the cycles of the dependency graph.(iv)When an initial state , then is called a transient point of or a transient state of the system. In the case of a finite space state, these points are also called eventually fixed points (resp., eventually periodic points) if their orbits finally arrive in a fixed point (resp., periodic orbit).(v)If there does not exist any state , such that , then is said to be a Garden-of-Eden (GOE) of . That is, is a state without predecessors. The set of GOEs of is denoted by .
According to [6, 72], one of the main goals in the study of a dynamical system is to give a complete characterization of its orbit structure. In this particular case, as the state space of the system is finite, every orbit is periodic or eventually periodic. This means that the evolution from any initial state always reaches an attractor . If the attractor consists of one state, it is called an attractor point, whereas if it consists of two or more states, it is said to be an attractor cycle. All the states that flow towards an attractor constitute what is called its basin of attraction. The time that an initial state takes to reach an attractor is named its transient.
The representation of the orbit structure of a system is called phase portrait, phase diagram, or transition diagram of the system. The phase portrait, when possible, gives a complete visual idea of the asymptotic behavior of the system from any initial state.
Example 7. In the next simple example, the different notions related to PDS that were introduced before will be studied in order to clarify them. Let us consider the PDS withover the directed graph shown in Figure 1, where the time-one-map of the system is given by the following lookup table (see Table 1).
This lookup table corresponds to the application of the maxterm as evolution operator, and the transition diagram or phase portrait of the system is the one in Figure 2.
Note that the system is not invertible, since there are 3 initial states that have not any predecessor: 001, 010, and 100. They constitute the GOE of the system and all of them are in the basin of attraction of the attractor cycle . In fact, they are predecessors of the state . Thus, these three states are eventually 2-periodic and the transient to arrive in the attractor cycle is equal to one.
On the other hand, the system does not present any fixed point, although it has a 3-periodic orbit . As expected, the Sharkovsky order  for the periodic orbit structure of a discrete dynamical system where the evolution operator is a continuous function of the interval does not work here.
The absence of fixed points and the presence of periodic orbits of period greater than two in this kind of PDS over directed dependency graphs are two important breakpoints with respect to the pattern followed by those defined over undirected graph (see ), as explained in .
As shown before, in spite of their relation with classical dynamical systems, the theory and analysis of PDS (and SDS) are based on techniques from algebra, combinatorics, and discrete mathematics in general. In fact, due to the especial discrete per excellence character of PDS, some dynamical topics, used to measure the intrinsic stability of a dynamical system, as sensitivity dependence on initial conditions or chaos [4, 72], are difficult to be translated into this context, since they are established according to the topological properties of the corresponding metric state space.
2.2. Comparative Study with Related Topics
In the specific literature, several topics which are related to PDS appear. In this section, an overview about them is given. The list of related topics presented is reduced to the most similar ones. The purpose is only to provide an introduction in order to compare them with PDS. Specifically, the nearest concepts, namely, cellular automata (CA) [21–25] and Boolean networks (BN) [14, 28], are reviewed. Other topics related to PDS are, for instance, finite-state machines [74, 75] and Petri nets [76–78], but they will not be discussed here.
2.2.1. Cellular Automata
Following Wolfram , a CA consists of a regular uniform lattice (or array), usually infinite in extent, with a discrete variable at each site called cell. The state of a CA at a time is completely determined by the state value of every cell at such time . The evolution of this system consists in the discrete time updating of the state value of its cells in a synchronous manner. The updating of the state value of each cell depends on a local function defined on the state values, at the previous time step, of other cells in its neighborhood. The neighborhood of a site in the uniform lattice is typically constituted by the site itself and all the immediately adjacent sites. These immediately adjacent cells are determined by the lattice structure and a homogeneous rule for the selection of neighbors.
Thus, CA can be contemplated as a particular case of (infinite) PDS (the concept of PDS can be easily extended to involve an infinite number of entities), if one considers cells as entities. Nevertheless, CA have a more restricted and uniform structure and, in contrast to PDS, CA are usually considered over inf of the system are fixed mple, , where is said to be the dimension of the CA.
The more restricted and uniform structure of CA is reflected by both the neighborhood of any cell and the local functions acting on it, which are determined homogeneously. Certainly, in classical CA, every cell has a neighborhood which is some sequence of lattice sites, which displays a homogeneous structure. If the CA is defined over , this homogeneous structure in the neighborhood of any cell is obtained by means of a (finite) subset which provides a rule to haveObserve that in a CA over an infinite structure like , any state of the system is given by a sequence , usually named a configuration. At this point, one can state the definition of a -dimensional cellular automata, taking into account the (finite) set of the state values that any cell can have and the rule to define the neighborhood of any cell over an infinite lattice , where the same local function is applied. When the set is equal to the basic Boolean algebra the CA is said to be a Boolean cellular automata.
One-dimensional (Boolean) CA with two possible values for the state of the cells, 0 and 1, in which the neighborhood of a given site is simply the site itself and the sites immediately adjacent to it on its left and right, are studied in . In such a case, the subset to get a homogeneous neighborhood is , and it is called a three-neighborhood structure (see Figure 3).
For two-dimensional CA, two of the most used neighborhood structures are the von Neumann neighborhood  and the Moore neighborhood  (see Figures 4 and 5). The finite subset to obtain the von Neumann neighborhood is while the one to get the Moore one is
CA over finite lattices with sites can also be defined. Two traditional ways to do that are as follows:(i)by imposing periodic boundary conditions, which consists in identifying the cells separated by positions, giving a CA over , an idea which can be extended to higher dimensions, giving a CA over a -dimensional tori,(ii)by imposing zero boundary conditions, which consists in using a line graph as a lattice and adding two additional extreme vertices whose states are always equal to zero.
Local rules for the updating of Boolean CA are in fact Boolean functions of variables, where is the number of neighbors in any neighborhood . Most of the research on CA has been concerned with one-dimensional cases, which are commonly known as elementary cellular automata (see [10, 12, 21–23, 30, 49, 71, 79, 81–83]).
For a one-dimensional, three-neighborhood, Boolean CA, a local function has to determine the (output) state value of the central cell, from the (input) state values of the central cell and its two immediately adjacent ones. As we are considering Boolean values, there are only possible input state values for any neighborhood in the lattice, namely,
As from any input state values the output state value can be 0 or 1; there are distinct local rules for elementary three-neighborhood CA. Traditionally, each of these local rules has a decimal number belonging to assigned and it is known as the Wolfram enumeration of elementary CA rules. The procedure to enumerate such rules consists in considering the (output) value states of the triples above in that order and then translating the corresponding binary number into a decimal one. That is, if one has the output value states of the triples above as in Table 2, then the number of the local rule isThis enumeration procedure can be generalized to other classes of neighborhoods in CA.
Thus, the elementary CA rule 0 erases any initial configuration, while 204 does not change any initial configuration; that is, it corresponds to the identity transformation. One of the most important elementary three-neighborhood CA is the one given by the local function , that is, the elementary CA rule number 150 (see Table 3).
CA, when finite, can be considered as a special kind of PDS by considering cells as entities. It is easy to understand that the lattice of a CA allows us to infer a PDS dependency graph by considering the cells as entities, being any entity adjacent to those that are its neighbors in the lattice. Even, when infinite, CA can be considered PDS, since the concept of PDS can be easily extended for an infinite number of entities.
Also, CA are updated in a parallel or synchronous manner by applying local functions on a subset that contains the (state value of the) cell. Nevertheless, in the last few years some extensions of the concept of CA considering sequential or asynchronous updating have appeared in the literature (see [84–86]). In fact, the concept of sequential dynamical system [9, 33–35, 40, 44, 45] constitutes a generalization of such CA extension.
However, CA are restricted cases of PDS in several ways. First of all, for a CA seen as a PDS, the dependency graph, which is derived from the lattice and the neighborhood structure, is regular, whereas the graph of a general PDS can be arbitrary. Secondly, CA have the same local function or rule associated with every cell, while general PDS can have distinct local functions to update different entities, which can be the restriction of a global one (see [9, 41]) or independently defined (see ). Thus, general PDS can have more involved update schemes.
Although theoretically the set of state values of any cell can be whichever finite set, most of works in the literature focus on Boolean CA [10, 12, 21–23, 30, 49, 71, 79, 81–83], where . In  and more recently in , a first approach in the study of CA where is any finite set is given. On the other hand, in , we study completely the problem when the set is any Boolean algebra in the more general context of PDS.
Summing up, PDS are a generalization of CA, where the state value of each entity is not affected necessarily by its neighbors (see Figure 6), but potentially by any entity in the network, and the updating function associated with an entity can be given by any arbitrary local function.
2.2.2. Boolean Networks
A Boolean network (BN) of size and connectivity consists of interconnected vertices, each one having inputs. The update of the state of any vertex is determined by the (directed) dependency relations and local rules which are given by Boolean functions. Hence, BN are a generalization of (finite) Boolean CA but, at the same time, a particular case of PDS by considering nodes as entities. One of the main differences with CA is that, in BN, the state of each node is not affected necessarily by its neighbors but potentially by any node in the network. Thus, the uniform structure of neighborhood in CA disappears. Nevertheless, some homogeneity remains, since each node is affected by connections with other (or the same) entities. This homogeneity makes BN a particular case of PDS where the connections can be totally arbitrary (see Figure 6).
Another important difference with CA is that local Boolean functions of -variables are generated randomly, which provides different lookup tables for each entity for the updating process. This idea has been carried out and extended for PDS in this work in two directions. Firstly, as can be seen in , local Boolean functions acting on each entity can have different number of variables (which cannot occur for BN); and secondly, they can be totally independent for each entity.
BN with nodes and connectivity are known as models or Kauffman models, since they were originally developed by him in  when modeling regulatory networks of (on-off) genes. For this reason, nodes of BN are often called genes. In particular, Kauffman models on complete networks are usually called random maps.
As in PDS, the state space is finite with possible states. Therefore, whichever the initial state is, eventually a state will be repeated. However, the number of models, when and are fixed, is lower than the number of possible dependency graphs that determine a PDS.
As for the case of CA, originally the updating of BN was synchronous, but, also in this case, several recent works provide some extensions of the concept, considering asynchronous updating schedule [11, 85, 86]. Also in this case, the concept of sequential dynamical system constitutes a generalization of such asynchronous Boolean networks.
Although first Kauffman models were deterministic, in the last decade some authors have studied an extension of the concept considering a probabilistic updating schedule [14, 66, 89]. In this context, they are named probabilistic Boolean networks and cannot be described from the perspective of dynamical systems, with Markov chains being the natural framework to set out their evolution. This idea of extension was provided for the first time in  and it will be explained in the following paragraphs.
Let , a gene or node of a BN, and denote by a local function to update the state value of for all . On the other hand, let be another local function related to the updating of the node . If any node is updating by means of with probability and by means of with probability , then a basic probabilistic BN is obtained. Thus, this stochastic construction may be viewed as a weighted superposition of two deterministic BN, that is, the one obtained with the global evolution operator and the other one achieved with the time-one-map . That is, by updating the BN with the evolution operator , a transition diagram could be determined, namely , while updating with another transition diagram, namely , could be also obtained. Then, by superposing the transition diagrams (which are in fact direct graphs), one can construct a digraph, assigning the weight to the arcs which are only in , the weight to the arcs which are only in , and the weight 1 to the arcs which are in both diagrams. This fused weighted digraph is often called the probabilistic phase space and its weighted matrix can be considered as the corresponding Markov chain matrix. The (probabilistic) evolution of the BN from an initial state can be then identified with a random walk over this probabilistic phase space that starts at this initial state of the BN.
The construction explained before is basic in the sense that only two kinds of functions with two probabilities are considered to update the nodes of the BN. Obviously, more functions could be considered and then the probabilistic phase space would be the result of the superposition of all the transitions diagrams obtained deterministically with any global operator, considering for any arc that appears in more than one of this transitions diagrams the weight resulting of the sum of all the probabilities assigned to any of the diagrams where this arc appears.
Finally, observe that the PDS notion could be extended in order to capture probabilistic updating in the same sense that probabilistic BN do. Nevertheless, in this case, they would lose the structure of (deterministic) dynamical systems.
3. Analytical Results on the Orbital Structure of PDS
3.1. PDS over Undirected Dependency Graphs
This section is devoted to review the orbit structure of parallel discrete dynamical systems with maxterms and minterms Boolean functions as global evolution operators. As a result, it is shown that the orbit structure does not remain when the system is perturbed.
In , it is proved that, for a parallel dynamical system associated with the maxterm , all the orbits of the system are fixed points or eventually fixed points. More precisely, the system has exactly two fixed points and the maximum number of iterations needed by an eventually fixed point to reach the corresponding fixed one is at most as large as the diameter of the dependency graph.
Dually, for a parallel dynamical system associated with the minterm , all the orbits of the system are fixed points or eventually fixed points, and the system has exactly two fixed points and the maximum number of iterations needed by an eventually fixed point to reach the corresponding fixed one is at most as large as the diameter of the dependency graph.
Observe that the orbit structures of and do not depend on the dependency graphs over which they are defined, covering all the possible casuistry of them.
In , it is proved that, for a parallel dynamical system associated with an arbitrary maxterm , all the periodic orbits of this system are fixed points or 2-periodic orbits, while the rest of the orbits are eventually fixed points or eventually 2-periodic orbits. In fact, from the demonstration, the following can be deduced.(i)If all the variables are in their direct form, only fixed points or eventually fixed points can appear.(ii)If all the variables are in their complemented form, only 2-periodic orbits or eventually 2-periodic orbits can appear.(iii)Otherwise, both kinds of situations coexist.
As before, dually, for a parallel dynamical system associated with an arbitrary minterm , all the periodic orbits of this system are fixed points or 2-periodic orbits, while the rest of the orbits are eventually fixed points or eventually 2-periodic orbits.
These results provide a complete characterization of the orbit structure of parallel discrete dynamical systems with maxterms and minterms Boolean functions as global evolution operators. In particular, one can infer that when other maxterms (resp., minterms) different from (resp., ) are considered as global functions to define the evolution of a system, the dynamics can change, giving as a result a certain absence of structural stability of the system so perturbed.
3.2. PDS over Directed Dependency Graphs
In this section, the orbit structure of parallel discrete dynamical systems over directed dependency graphs (PDDS) (the abbreviation PDDS will be written for the singular and plural forms of the corresponding term, since it seems better from an aesthetic point of view) with Boolean functions as global evolution operators is reviewed. In this sense, for the cases corresponding to the simplest Boolean functions and , it is explained that only fixed or eventually fixed points appear, as occurs over undirected dependency graphs. However, for general Boolean functions, it is shown that the pattern found for the undirected case breaks down.
In , it is proved that for a parallel dynamical system over a directed dependency graph associated with the maxterm , all the orbits of this system are fixed points or eventually fixed points. Dually, for a parallel dynamical system over a directed dependency graph associated with the minterm , we have also that all the orbits of this system are fixed points or eventually fixed points.
Nevertheless, comparing with the undirected case studied in the previous section, for the directed one the number of fixed points can increase significantly, except if the digraph is strongly connected (i.e., if for all pairs of vertices , there exists a path from to and another one from to ), where only two equilibrium states are possible, namely, the state with all the entities activated and the one with all the entities deactivated. In general, this number of possibilities depends on the structure of the directed dependency graph of the system and it is not possible to give a generic result as in the undirected case.
Besides, also in , it is demonstrated that for the general case of PDDS associated with maxterms and minterms as global evolution operators any period can appear, breaking the pattern found for the undirected case where only fixed points or 2-periodic orbits can exist . In order to prove that, a method is developed to provide, for every given period, a PDDS which presents a periodic orbit of such a period.
Subsequent research  allows us to check that this breakdown is due to the existence of directed cycles in the dependency digraph, while for PDDS over acyclic dependency digraphs the periodic orbits continue being only fixed points or 2-periodic ones. Moreover, the orbit structure of special digraph classes as line digraphs, arborescences, and star digraphs is similar and it is possible to specify the number of iterations needed by an eventually periodic point to reach the corresponding periodic orbit.
However, it is not easy to control the orbit structure when the dependency graph has cycles. In such a case, the orbital structure depends on both the global evolution operator and the structure of the digraph. In particular, in , it is shown that over circle digraphs can present periodic orbits of any period except fixed points and periods 4 and 6, and the same occurs for .
4. Extensions of the Concept of PDS
In this section, two extensions on parallel dynamical systems are reviewed. The first one is related to the manner of defining the evolution update and the second one to consider that the states of the entities can take values in an arbitrary Boolean algebra.
4.1. PDS on Independent Local Functions
When Kauffman introduced BN for the first time , he contemplated the possibility of updating any entity using independent local functions. This idea is extended to PDS in , since Boolean functions acting on each entity can have different number of variables (which cannot occur for BN) due to the random (connections) structure of the dependency graph. This extension of the update method widely generalizes the traditional one, where only a global Boolean function is considered for establishing the evolution operator of the system. Besides, this analysis allows us to show a richer dynamics in these new kinds of parallel dynamical systems.
In particular, for a PDS over an undirected or directed graph where each local function is either or , it is demonstrated that all the orbits of this system are fixed points or eventually fixed points. Although the orbit structure coincides with the corresponding one for a PDS where the updating global Boolean function is either or , for a PDS as the one described above, the number of fixed points of the system (and therefore the complexity of the system) may increase depending on the election of the local functions and the structure of the dependency graph. In order to illustrate this, we show the following example.
Example 8. Consider the undirected dependency graph given by and . If we consider the PDS defined by the global Boolean function (resp., ), then the fixed points are and . On the other hand, if we consider the PDS defined by the local functionsthen the fixed points are , , , , and .
Moreover, for a PDS over an undirected graph where each local function is either or , the periodic orbits are fixed points or 2-periodic orbits and the only possible fixed point is the one where if and if . Nevertheless, a PDS over a directed graph where each local function is either or can present periodic orbits of any period .
Finally, also in , it is demonstrated that for a PDS over an undirected graph where each local function , the periodic orbits of this system are fixed points or 2-periodic orbits, while over a directed graph periodic orbits of any period can appear.
4.2. PDS with General Boolean State Values
In , another extension of PDS is provided, by considering that the states of the vertices can take values in an arbitrary Boolean algebra of elements, , . This situation naturally appears, for instance, when each entity is composed of subentities which can be activated or deactivated, and consequently the states of the entities belong to the Boolean algebra . This definition widely extends the traditional one where it is assumed that every entity can take values in the simplest Boolean algebra . To study the orbit structure of these more general PDS, in view of the Stone’s Theorem (see ), an isomorphism as Boolean algebras of and is considered. Then, since the operations and complement elements are obtained coordinate by coordinate in , it is shown how one can translate all the results previously achieved to this new context.
5. Algorithms for the Computation of Orbits
As said before, in , it is demonstrated that for the general case of PDDS associated with maxterms and minterms as global evolution operators any period can appear, breaking the pattern found for the undirected case. Since we have a finite state space, it is obvious that every orbit is either periodic or eventually periodic. However, it is not so easy to determine a priori the different coexistent periods of its orbits, because it depends fundamentally on the structure of the directed dependency graph. In this sense, in , algorithms for computing the (eventually) period of any orbit are given.
Since the coexistence of periodic orbits depends on the number of entities, their connections, and the Boolean operator, several matrix algorithms for the computation of orbits in PDDS over directed dependency graph are developed in . These algorithms constitute a new tool for the study of orbits of these dynamical systems. The algorithms are also valid for PDS over undirected dependency graphs. Likewise, the methods are extended to the case of SDS, even when the local functions that are considered for the updating of any entity are independent.
These matrix methods of computation of orbits provide an especial mathematical linearization of the system, by means of particular products of the adapted adjacency matrix of the dependency graph and the state vectors.
We think the algorithms established can help to infer some analytical results on the orbit structure of PDS and SDS over special classes of digraphs, since they can be used to reveal some orbit patterns.
6. Conclusions and Future Research Directions
The results on PDS reviewed provide a full characterization of the orbit structure of PDS over undirected graphs covering all the casuistry, since it studies the behavior of any parallel dynamical system regardless the type of graph, the number of entities, and the relationships among them. Specifically, when the evolution functions are or or a combination of both, the orbits are fixed points. But when considering other maxterm or minterms dynamics can change, due to the appearance of 2-periodic orbits, resulting in a certain absence of structural stability when systems are slightly perturbed.
On the other hand, essential properties of the orbit structure of PDS over directed dependency graphs are achieved. In fact, evolutions with and continue giving only fixed or eventually fixed points. But, on the contrary, for general maxterm or minterm functions, it is shown that any period can appear, so breaking the pattern found for the undirected case. Actually, several PDS over especial digraph class are characterized. Besides, it is shown that this disappearance of the general pattern is due to presence of cycles in the dependency digraph.
Motivated by these findings, computation methods for calculating orbits in PDS are also developed. Besides, two extensions of the PDS concept are shown.
The study developed so far opens several future research directions. Most of the problems that can arise after this study of PDS correspond to extensions of the model, some of which have already begun to be studied in some works in the narrower context of CA or BN. It would be interesting to extend this study to the more general context of PDS (or SDS).
One of the initial questions posed by this work is to check if the results for PDS are transferable to the case of SDS. Paper  shows a first approach to the problem, since the authors carried out a parallel study of the orbital structure of PDS and SDS with evolution and operators, obtaining similar results in both cases, that is, fixed points when the evolution operator is and periodic points of period 2 when the evolution operator is . By duality, these results are also valid for the case of and operators.
Results in  solve the problem of the orbital structure of PDS over undirected graphs for which the evolution operator is any maxterm or minterm, including the former , , , and . Therefore, a possible new line of research in view of this work is to study if the results obtained for PDS can be translated to the case of SDS, fundamentally in the case in which the systems are defined over undirected dependency graphs.
In the case of directed dependency graphs, the extension of the results to SDS seems much more complicated, as one can check with simple examples.
When modeling discrete processes, it can occur that the entities are updated in a mixed manner between parallel and sequential one. This happens when there exists a partition of the set of entities of the system such that every set of this partition consists of entities that are updated synchronously, but the updating of these sets is asynchronous. That is, once the partition of the vertex set is known, the subsets can be ordered by a permutation that indicates how these subsets are sequentially updated. This means that given an ordered partition , firstly the entities of the subset are updating in a parallel manner; then the entities of are also updated in a parallel manner, but taking into account the new states values of the entities in , and so on. These systems can be called mixed dynamical systems (MDS). This new updating scheme could be used as an intermediate step between PDS and SDS. In fact, SDS can be considered as MDS where the partition of the set is given by all the subsets with only one element. This notion should be investigated not only for the natural interest to model mixed computer processes, but also for the possible repercussions in the translation of results from PDS to SDS.
The original definition of CA  contemplates the possibility that the cells take state values in a finite set, although subsequently the majority of studies have been made in the case of Boolean CA. The studies that have appeared so far in this direction ([87, 88]) consider some uniformity, since every entity can have all the state values in the finite set. This can have the physical sense of assuming that the entities can have different levels of intensity. In this sense, considering that the entities of a system may not have necessarily the same number of states, it could be encouraging to establish a consistent update schedule when the states of the different entities of the system do not belong to the same finite set.
In , we study completely the problem when the set is any Boolean algebra in the more general context of PDS. This constitutes an important issue, since it can model the case where entities are composed of a finite number of parts that can be activated or deactivated. In this context, the problem above concerning entities with different set of state values is maybe more suitable, since any set with structure of Boolean algebra is isomorphic to a subalgebra of and then one could establish a surjection from this algebra to one smaller in order to make a correspondence among states of this bigger Boolean algebra and the smaller one.
On the other hand, to give a fair explanation about how a system evolves when the entities can have state values in a finite set would open the door to the definition and posterior study of a fuzzy version of PDS.
Of course, one can also think in the possibility of considering an infinite set of state values for the entities. But, again, this is not a natural situation in the processes modeled by PDS.
Also in the original definition of CA in , it is allowed to have an infinite number of cells in the system. Although computer processes depend on a finite number of entities, the concept of PDS and the results obtained in this work can be extended to the case of infinite number of entities.
In the same sense as before, one can also think in the possibility of considering an infinite number of entities and a finite or infinite set of state values for the entities.
The notion of dynamically equivalent systems allows us to describe theoretically the class of all dynamically equivalent systems. Therefore, it should be desirable to find out some properties in the dependency graph and the evolution operator which provide or characterize PDS equivalent to a given one. A first approach in this sense appears in , where finite sequential dynamical systems on binary strings are studied. Several equivalence relations notions as isomorphic and stably isomorphic are introduced, and the resulting equivalence classes are studied. In this same sense, researching in the conditions that provoke bifurcations is a compiling need.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work has been partially supported by Grants MTM2011-23221.
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