Boolean network
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A Boolean network consists of a discrete set of boolean variables each of which has a Boolean function (possibly different for each variable) assigned to it which takes inputs from a subset of those variables and output that determines the state of the variable it is assigned to. This set of functions in effect determines a topology (connectivity) on the set of variables, which then become nodes in a network. Usually, the dynamics of the system is taken as a discrete time series where the state of the entire network at time t+1 is determined by evaluating each variable's function on the state of the network at time t. This may be done synchronously or asynchronously.^{[1]}
Boolean networks have been used in biology to model regulatory networks. Although Boolean networks are a crude simplification of genetic reality where genes are not simple binary switches, there are several cases where they correctly capture the correct pattern of expressed and suppressed genes.^{[2]}^{[3]} The seemingly mathematical easy (synchronous) model was only fully understood in the mid 2000s.^{[4]}
Contents
Classical model[edit]
A Boolean network is a particular kind of sequential dynamical system, where time and states are discrete, i.e. both the set of variables and the set of states in the time series each have a bijection onto an integer series. Such systems are like cellular automata on networks, except for the fact that when they are set up each node has a rule that is randomly chosen from all 2^{2K} possible ones with K inputs. With K=2 class 2 behavior tends to dominate. But for K>2, the behavior one sees quickly approaches what is typical for a random mapping in which the network representing the evolution of the 2^{N} states of the N underlying nodes is itself connected essentially randomly.^{[5]}
A random Boolean network (RBN) is one that is randomly selected from the set of all possible boolean networks of a particular size, N. One then can study statistically, how the expected properties of such networks depend on various statistical properties of the ensemble of all possible networks. For example, one may study how the RBN behavior changes as the average connectivity is changed.
The first Boolean networks were proposed by Stuart A. Kauffman in 1969, as random models of genetic regulatory networks^{[6]} but their mathematical understanding only started in the 2000s.^{[7]}^{[8]}
Attractors[edit]
Since a Boolean network has only 2^{N} possible states, a trajectory will sooner or later reach a previously visited state, and thus, since the dynamics are deterministic, the trajectory will fall into a steady state or cycle called an attractor (though in the broader field of dynamical systems a cycle is only an attractor if perturbations from it lead back to it). If the attractor has only a single state it is called a point attractor, and if the attractor consists of more than one state it is called a cycle attractor. The set of states that lead to an attractor is called the basin of the attractor. States which occur only at the beginning of trajectories (no trajectories lead to them), are called gardenofEden states^{[9]} and the dynamics of the network flow from these states towards attractors. The time it takes to reach an attractor is called transient time.^{[4]}
With growing computer power and increasing understanding of the seemingly simple model, different authors gave different estimates for the mean number and length of the attractors, here a brief summary of key publications.^{[10]}
Author  Year  Mean attractor length  Mean attractor number  comment 

Kauffmann ^{[6]}  1969  
Bastolla/ Parisi^{[11]}  1998  faster than a power law,  faster than a power law,  first numerical evidences 
Bilke/ Sjunnesson^{[12]}  2002  linear with system size,  
Socolar/Kauffman^{[13]}  2003  faster than linear, with  
Samuelsson/Troein^{[14]}  2003  superpolynomial growth,  mathematical proof  
Mihaljev/Drossel^{[15]}  2005  faster than a power law,  faster than a power law, 
Stability[edit]
In dynamical systems theory, the structure and length of the attractors of a network corresponds to the dynamic phase of the network. The stability of Boolean networks depends on the connections of their nodes. A Boolean network can exhibit stable, critical or chaotic behavior. This phenomenon is governed by a critical value of the average number of connections of nodes (), and can be characterized by the Hamming distance as distance measure. In the unstable regime, the distance between two initially close states on average grows exponentially in time, while in the stable regime it decreases exponentially. In this, with "initially close states" one means that the Hamming distance is small compared with the number of nodes () in the network.
For NKmodel^{[16]} the network is stable if , critical if , and unstable if .
The state of a given node is updated according to its truth table, whose outputs are randomly populated. denotes the probability of assigning an off output to a given series of input signals.
If for every node, the transition between the stable and chaotic range depends on . The critical value of the average number of connections is .^{[17]}
If is not constant, and there is no correlation between the indegrees and outdegrees, the conditions of stability is determined by ^{[18]}^{[19]}^{[20]} The network is stable if , critical if , and unstable if .
The conditions of stability are the same in the case of networks with scalefree topology where the inand outdegree distribution is a powerlaw distribution: , and , since every outlink from a node is an inlink to another.^{[21]}
Sensitivity shows the probability that the output of the Boolean function of a given node changes if its input changes. For random Boolean networks, . In the general case, stability of the network is governed by the largest eigenvalue of matrix , where , and is the adjacency matrix of the network.^{[22]} The network is stable if , critical if , unstable if .
Variations of the model[edit]
Other topologies[edit]
One theme is to study different underlying graph topologies.
 The homogeneous case simply refers to a grid which is simply the reduction to the famous Ising model.
 Scalefree topologies may be chosen for Boolean networks.^{[23]} One can distinguish the case where only indegree distribution in powerlaw distributed,^{[24]} or only the outdegreedistribution or both.
Other updating schemes[edit]
Classical Boolean networks (sometimes called CRBN, i.e. Classic Random Boolean Network) are synchronously updated. Motivated by the fact that genes don't usually change their state simultaneously,^{[25]} different alternatives have been introduced. A common classification^{[26]} is the following:
 Deterministic asynchronous updated Boolean networks (DRBNs) are not synchronously updated but a deterministic solution still exists. A node i will be updated when t ≡ Q_{i} (mod P_{i}) where t is the time step.^{[27]}
 The most general case is full stochastic updating (GARBN, general asynchronous random boolean networks). Here, one (or more) node(s) are selected at each computational step to be updated.
 The PartiallyObserved Boolean Dynamical System (POBDS)^{[28]}^{[29]}^{[30]}^{[31]} signal model differs from all previous deterministic and stochastic Boolean network models by removing the assumption of direct observability of the Boolean state vector and allowing uncertainty in the observation process, addressing the scenario encountered in practice.
Application of Boolean Networks[edit]
Classification[edit]
 The Scalable Optimal Bayesian Classification^{[32]} developed an optimal classification of trajectories accounting for potential model uncertainty and also proposed a particlebased trajectory classification that is highly scalable for large networks with much lower complexity than the optimal solution.
See also[edit]
References[edit]
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External links[edit]
 DDLab
 Analysis of Dynamic Algebraic Models (ADAM) v1.1
 RBNLab
 NetBuilder Boolean Networks Simulator
 Open Source Boolean Network Simulator
 JavaScript Kauffman Network
 Probabilistic Boolean Networks (PBN)
 A SATbased tool for computing attractors in Boolean Networks
 CoLoMoTo (Consortium for Logical Models and Tools)