1 Introduction
In recent years, there has been a resurgence in the use of Evolutionary Algorithms (EAs) for datadriven modelling of dynamical systems. Undoubtedly, one of the main driving forces for this is the steady growth of computation power. EAs are being increasingly used in a multitude of engineering domains and life science (Eiben et al., 2003; AriasMontano et al., 2012). Across several domains, EAs have generated results that are competitive and, sometimes, even surprising (Eiben et al., 2003). Another factor contributing to the growing popularity of EAs is that these algorithms can be used to generate solutions for complex problems for which no systematic solution approach exists in general. In parametric system identification, the estimation of model structure and model complexity is one such problem.
Model structure selection is a classical problem in system identification. Over the years, a variety of methods for system identification have been developed. Each of these methods adopt different approaches to solve the problem of model structure selection. While methods like Prediction Error Minimization (PEM) treat model structure selection as a user’s choice (Ljung, 1999), other methods (for example, Pillonetto et al. (2011)) rely on a flexible model structure, and attempt to estimate or control the complexity of the modeltobeestimated via regularization. Furthermore, the appropriate model complexity is often chosen by ranking models based on an information metric, such as AIC, BIC, or based on a userdefined complexity measure (Rojas et al., 2014). In cases where the number of candidate models grows combinatorially with respect to the length (or the complexity) of the model, a rankingbased complexity selection strategy becomes intractable, restricting model structure selection to regularization or shrinkage based methods.
As a consequence of the aforementioned challenges, heuristicsbased methods such as EAs have been used to estimate model structure and complexity, with a fair amount of success. However, the application of EAs have been, to some extent, superficial. The premise of the biologicallyinspired heuristics used in EAs is that the solutions of a given problem can be constructed from fundamental building blocks, and these fundamental components can be interchanged between different solutions. In the system identification literature, the proposed EAbased approaches to model structure and complexity selection can be categorized as follows:

approaches that choose a fixed model structure and use EAs to determine the appropriate model complexity (or model terms), and

approaches that use EAs to explore model structure and model complexity.
In the first category of EAbased approaches, the basic building blocks of an EA are chosen such that only models with a specific model structure can be generated. Hence, these approaches cannot be typically extended to other model structures without significant modifications. This approach can be found in Fonseca and Fleming (1996); RodriguezVazquez et al. (2004); RodríguezVázquez and Fleming (2000), where the authors use EAs to perform term selection within a chosen model structure. This approach is also used in Kristinsson and Dumont (1992), where the authors use GAs to estimate polezero locations for ARMAX models.
In the second category of EAbased approaches, more generic set of building blocks are used in the EA, allowing the generation of models with arbitrary model structures. In this case, EAs are used to determine not just the appropriate complexity of the model, but also the approprite model structure (e.g., in terms of the nonlinear functions to be included in the model). However, unrestrained generation of arbitrary model structures using EA may result in models that are not wellposed, e.g., models with discontinuities, noncausality, or finite escapetime. Typically, these problems are avoided by using arbitrary adhoc solutions, e.g, setting all discontinuities to 0. Another common drawback of EA based approaches that fall in the second category is that prior knowledge of the dynamical system cannot be incorporated systematically in the identification procedure. In Madár et al. (2005), the authors use GP to identify NARMAX models that may contain arbitrary nonlinearities. While the authors are interested in models that are linearintheparameters, GP may return models that do not belong to that class. Consequently, the authors use an adhoc solution to ensure that the candidate model structures generated by GP are linearly parameterized. A similar approach was used in Quade et al. (2016) with a larger set of mathematical operations. Again, the proposed approach does not allow for systematic inclusion of model structure constraints or prior knowledge of the system. A slightly different approach is used in Gray et al. (1998), where the authors use GP to construct linear or nonlinear models from basic elements like SIMULINK blocks and static nonlinearities. Again, the combination of various SIMULINK blocks cannot be systematically structured to avoid illposed models.
In this paper, we propose a generative grammar based representation of stochastic parametric dynamical systems. The proposed representation allows for the generation of complex, yet wellposed dynamical models by combining a set of fundamental building blocks in wellspecified ways. The resulting generative declaration of models defines a notion of model set that is more generalized than that conventionally used, for example, in Ljung (1999). The generative grammar used in this work is called Tree Adjoining Grammar (TAG) (Joshi and Schabes, 1997). The use of TAG in an EAbased approach makes it possible to develop a system identification framework where EAs are used to automatically determine the structure and complexity of a model from a generic, wellposed class of dynamical models, while systematically incorporating model structure constraints and prior knowledge. A preliminary concept of the proposed framework (without proofs) was presented in Khandelwal et al. (2019). The proposed approach for grammarbased identification was found to produce results that were comparable to stateoftheart nonlinear system identification approaches, while using no specialized knowledge of the benchmark system being identified.
The main contributions of this paper are the following. We present a detailed discussion on the discretetime inputoutput representation of dynamical systems using TAG, and introduce a new notion of a model set defined by the generative capacity of a TAG. Subsequently, we develop a TAG for the polynomial NARMAX model class. We prove that any model structure generated by the proposed TAG belongs to the class of polynomial NARMAX models, and conversely, any polynomial NARMAX model can be represented using the proposed TAG (for which an algorithm is also proposed). We demonstrate that the model set corresponding to the proposed TAG includes, as special cases, other commonly used model structures such as FIR, ARX and Truncated Volterra series models. We also demonstrate that the proposed representation can be easily extended to other model structures (namely polynomial Nonlinear BoxJenkins, or NBJ). Note that, while the TAG based model set notion developed in this contribution is motivated by its applicability in an EA based identification methodology, the identification approach itself is not in the scope of the present contribution. A preliminary version of such an identification methodology can be found in Khandelwal et al. (2019) and Khandelwal et al. (2019).
The contributions in this paper differ from Khandelwal et al. (2019) in the following respects:

we formulate a TAG for a larger class of dynamical systems (the polynomial NARMAX class), and prove their equivalence,

we provide an algorithm to compute an equivalent TAG representation of a given polynomial NARMAX model,

we illustrate, via examples, the restriction (and generalization) of the proposed TAG in order to generate models with more specific (or generic) structures.
The remainder of the paper is structured as follows. The concept of TAG is introduced, both informally and formally, in Sec. 2. In Sec. 3 we introduce the notion of model set as defined by a given TAG, and propose a TAG that generates the class of polynomial NARMAX models. Several examples are used to illustrate the concept in Sec. 4, followed by concluding statements in Sec. 5.
2 Tree Adjoining Grammar
To set the stage for the development of TAG for stochastic nonlinear systems, first we introduce the basic concepts of TAG. Since TAG was initially developed from linguistic considerations, a linguistic example will be used to illustrate the methodology. This will be followed by formal definitions. To make the example illustrative, we first specify an example string, and then infer a TAG that would generate the given string. Conversely, for the formal definitions, we will begin with the basic components of a TAG and lead up to the definition of TAG and operations that can be performed on TAGs.
2.1 An informal description
Informally, a formal grammar can be described as a set of rules for generating strings. The resulting set of strings is called the language generated by the grammar. In contrast, TAG describes a set of rules for generating trees. The resulting set of trees is called the tree language of the TAG. The yield of all the trees in the tree set subsequently determines the corresponding language.
The following example has been derived from Joshi and Schabes (1997). Consider the sentence “A man saw Mary”. Simple grammatical constructs can be used to decompose the given sentence into its basic components. For example, the sentence consists of articles (“A”), nouns (“man”, “Mary”) and verbs (“saw”). Other underlining structures, such as subjects and predicates, can also be observed in the sentence. The sentence, together with the underlying grammatical structure can be represented in a single tree structure as shown in Fig. 1. The tree depicted in Fig. 1 is called a derived tree. The yield of a derived tree are the labels associated with the leaves of the tree. Hence, the yield of the derived tree in Fig. 1 is “A man saw Mary”.
The given derived tree can be obtained by combining basic building blocks that are constituents of the TAG. Fig. 2 depicts the set of initial trees and auxiliary trees , collectively known as elementary trees, that can be combined in specific ways to produce the derived tree in Fig. 1. The set of initial trees can be informally described as a set of nonrecursive replacement rules that can be used to generate a set of trees. The set of auxiliary trees can be described as a set of recursive replacement rules. Consequently, each auxiliary tree has a terminal node with the same label as that of its root node.
The downward arrow symbol and the star symbol in Fig. 2 represent nodes in a tree that are available for a substitution and adjunction operation respectively. A substitution operation can be used to substitute an initial tree into, for instance, another initial tree, if and only if the latter has a terminal node (leaf) with a label that matches the label of the root node of the prior. On the other hand, adjunction can be loosely described as the operation of inserting an auxiliary tree into a syntactic tree. Adjunction of an auxiliary tree can take place on a nonterminal node of a syntactic tree if and only if the node has a label that matches the label of the root node of the auxiliary tree to be adjoined.
Consider the following sequence of operations. The initial tree can be substituted in at the location of the “sub” node. Let’s denote the resulting tree as . The tree is an example of a syntactic tree, a tree obtained by applying an arbitrary number of substitution and adjunction operations to a given initial tree. Again, the initial tree can be substituted to the syntactic tree at the location of the “pred” node. Let the result be denoted as . Note that has the same structure as the example in Fig. 1, upto the last level of the derived tree, where specific articles, nouns and verbs are substituted in the tree to obtain the yield “a man saw Mary”. Substitution can be performed on a initial tree or syntactic tree as long as there exist nodes available for substitution, marked by . A derived tree is a syntactic tree in which none of the terminal nodes (leaves) are available for substitution. The initial and auxiliary trees provide an alternative representation, the derivation tree, as shown in Fig. 2(a). Based on the TAG in Fig. 2, more complex sentences can also be generated. For example, the auxiliary tree can be adjoined to the root node of since both root nodes have the label “sentence”. This operation effectively adds an adverb before the sentence, yielding the sentence “yesterday a man saw Mary”. The resulting derivation tree is depicted in Fig. 2(b).
The set of all derived trees that can be obtained, by starting from a given start symbol, say “sentence”, and applying an arbitrary number of adjunctions and/or substitutions using elementary trees is called the tree language of the corresponding TAG. The string yield of all trees in the tree set is called the string language of the corresponding TAG.
We can now introduce the formal definitions of the concepts that were informally described in this example.
2.2 The formal definitions
The formal definitions of TAG and related concepts can be found in Joshi and Schabes (1997) and Kallmeyer (2009). These definitions are reproduced here for completeness.
Definition 1.
A finite tree is a directed graph, denoted by , where, is the set of vertices, is the set of edges, and is the root node, such that

contains no cycles,

has indegree (number of incoming edges) 0,

All have indegree 1,

Every is accessible from ,

A vertex with outdegree (i.e., number of outgoing edges) 0 is a leaf.
Definition 2.
A labeling of a graph over a signature is a pair of functions and , with being a set of disjoint alphabets.
For the next definitions, assume and to be disjoint sets of nonterminals and terminals, respectively.
Definition 3.
A syntactic tree is an ordered, labelled tree such that the label for each vertex with outdegree at least 1 and for each leaf .
Definition 4.
An auxiliary tree is a syntactic tree such that there is a unique leaf , marked as foot node, with . An auxiliary tree is denoted as .
Definition 5.
An initial tree is a nonauxiliary syntactic tree.
With the basic ideas defined, we can now define TAG, and the related operations.
Definition 6.
A Tree Adjoining Grammar is a tuple , where

are disjoint alphabets of nonterminals and terminals,

is a start symbol,

is a finite set of initial trees and is a finite set of auxiliary trees.
The set of trees is called elementary trees.
Definition 7 (Substitution).
Let be a syntactic tree and be an initial tree and . The result of substituting into at node , denoted as , is defined as follows

If is not a leaf or is a foot node or , then is not defined,

otherwise, with
(1) and
(2)
The substitution operation is illustrated in Fig. 3(a).
Definition 8 (Adjunction).
Let be a syntactic tree and be an auxiliary tree and with outdegree at least 1. The result of adjoining into at node , denoted as , is defined as follows

if then is undefined,

else with
(3) and
(4)
The adjunction operation is illustrated in Fig. 3(b).
Recall that a tree obtained by performing an arbitrary number of valid substitution and adjunction operations to an initial tree with is called a derived tree (for example, as in Fig. 1). Also recall that the substitution and adjunction operations performed can be represented in a tree representation called derivation tree (for example, as in Fig. 3). A derived tree is said to be saturated if all leaves of the derived tree belong to the set and cannot be further substituted. The corresponding derivation tree is also said to be saturated.
Definition 9 (Tree language and string language).
Let be a TAG. The tree language of grammar is defined as the set of all saturated derived trees in with root .
The string language of is the set of yields of the trees in .
3 TAG Description of Dynamical Systems
In this Section, we define a notion of model set based on TAG and propose a TAG for a generic class of dynamical models  the polynomial NARMAX class.
3.1 Model set
Consider the following discretetime inputoutput representation of a nonlinear dynamical model
(5) 
where are the input and output signals at timeinstant , is a noise signal independent of input , constants and are the corresponding maximum timelags and the nonlinear function belongs to an arbitrary set of functions . In PEM, the set of functions , also known as the model set, along with a specified choice for and , is determined by a user based on expert knowledge, prior information and informative experiments. It will be demonstrated in Sec. 3.2 that TAG can be used to generate trees that yield nonlinear functions with desirable structural properties and varying choices of arguments (time lags of the involved and signals). This capability of TAG leads to a more generalized notion of model set . In order to formalize this concept, we introduce a function that maps from function to the righthandside expression in (5) (in string form). We can now define a new notion of model set, based on TAG, defined as follows.
Definition 10.
For a given TAG , the corresponding model set is defined as the set of models in the form of (5) such that .
Note that this is a more generalized notion of model set as compared to that used in PEM. In PEM, a model set is typically determined by choosing a fixed model structure along with a suitable parameterization (i.e. model complexity). On the other hand, in this work, the choice of initial and auxiliary trees of a TAG automatically determines the model set. The advantage of such a declaration of a model set is that, when no prior information is available, the model set can be chosen to span a number of commonly used model classes without a prior specification of the model complexity. On the other hand, when prior information on the structure or complexity of the model is available, the grammar can be suitably refined to restrict the model set. In the subsequent sections, we propose a TAG for a generic model class, and demonstrate that the resulting model set spans a number of model structures commonly used in PEM.
3.2 The polynomial NARMAX model class
The NARMAX model class is a flexible class on nonlinear inputoutput dynamical models Leontaritis and Billings (1985). The polynomial NARMAX model class is the set of all NARMAX models where the nonlinear relationships are of the polynomial kind. Polynomial NARMAX is a convenient model representation since any continuous function on a closed space can be approximated arbitrary well using polynomial functions (based on Weierstrass theorem, see Stone (1948)). Furthermore, the family of polynomial NARMAX models includes, as special cases, other commonly used model classes such as FIR and ARMAX. It will be shown that these models can be generated by suitably restricting the TAG presented here.
A discretetime SISO polynomial NARMAX model can be represented as (see Billings (2013))
(6) 
where is the order of the polynomial nonlinearity, are the model parameters, and
is a vector consisting of the past input, output and noise regressors as follows
(7) 
We will also use the following alternative and equivalent representation for polynomial NARMAX models:
(8) 
where is the number of model terms, are the model parameters, are the exponents for output, input and noise terms.
3.3 Proposed TAG representation
In this section we propose a TAG for the polynomial NARMAX model class. The proposed TAG captures the structural relationships in (8). In the sequel, the time index will be dropped in the context of the proposed TAG, as will be used to denote a backward time shift. For convenience, introduce the following notation. For a given model in the form of (8), define , and . For the model term, the sequence of delays in the input, noise and output factors are denoted by , , respectively.
Theorem 1.
For the first part of the proof, we show that for any polynomial NARMAX model in the form of (8), there exists a derivation tree such that the resulting derived tree has a yield that is equal to the RHS of (8). Algorithm 1 constructs such a derivation tree for a given polynomial NARMAX model. The procedure Delays adjoins auxiliary tree to the derivation tree at vertex . The algorithm constructs the derivation tree by introducing the first factor ( or ) of each of the model terms, and subsequently building each of the branches by introducing the remaining factors with the corresponding delays and exponents.
For the second part of the proof, it needs to be shown that all expressions in , i.e., yields of all possible trees generated by , are RHS expressions of polynomial NARMAX models. This is proven by structural induction. We first observe that the simplest tree in is the initial tree with the yield . This corresponds to the model
(9) 
which belongs to the polynomial NARMAX class. Now, consider an arbitrary saturated derived tree whose yield is the RHS of a polynomial NARMAX model. This implies that the yield is a polynomial expression in terms of the factors , and . To complete the principle of induction, it must be shown that any possible adjunction to results in a new tree in whose yield is also a polynomial expression in terms of the aforementioned factors.
For convenience, the auxiliary trees are grouped based on the operators involved  are called additivetype auxiliary trees, are called multiplicativetype, and is called delaytype auxiliary tree. The following adjunctions be made on :

adjunction of an additivetype tree. Such an adjuction introduces an input, output or noise term additively in the expression while respecting the causality of the expression. Hence the resulting expression is also a polynomial;

adjunction of a multiplicativetype tree. This simply introduces multiplicative factors to an existing model term, and hence, the resulting expression is also a polynomial;

adjunction of a delaytype tree. This operation simply adds delays to an existing monomial, and hence preserves the polynomial structure of the expression.
Since all possible operations yield a causal polynomial expression, it can be concluded that consists of only dynamical polynomial expressions in terms of the factors and which corresponds to a polynomial NARMAX model. This concludes the proof. ∎
Theorem 1 demonstrates that structural properties of a rich class of dynamical models can be captured within a compact set of trees of a TAG. The expansive representational capability of TAG can be exploited using EAs such as GP to identify models without prior specification of structure and complexity, as demonstrated in Khandelwal et al. (2019). Furthermore, Algorithm 1 provides a method to compute the derivation tree representation of a given polynomial NARMAX model in terms of grammar . Consequently, available prior information about the model of the system can be translated to TAG representation (or incorporated in tree sets ), thereby making the evolutionary search more efficient. Hence, the use of TAG enables identification within a larger class of dynamical models without requiring userinteraction, while simultaneously allowing the user to restrict the evolutionary search effectively.
4 Illustrations
In this section we discuss aspects of TAG useful for EAbased SI. We demonstrate the use of TAG to generate polynomial NARMAX models. It is also shown that models belonging to simpler model classes can be generated by scaling down the set of elementary trees of appropriately. Furthermore, more flexible model classes can be represented by scaling up the set of elementary trees. This is demonstrated by extending the proposed TAG to generate Nonlinear Box Jenkins (NBJ) models.
4.1 Model generation using
Three illustrative examples are used to demonstrate the generation of models using . The models generated belong to the ARX, polynomial NARX and polynomial NARMAX model classes. It will be demonstrated that by restricting the elementary trees and to subsets of the elementary trees in the proposed TAG , we can generate models that only belong to model subclasses that are properly included in the set of polynomial NARMAX models, such as FIR and truncated Volterra series.
4.1.1 ARX example
ARX models can be described by the equation
(10) 
where are coefficients. The grammar can be used to generate ARX models by restricting the auxiliary tree set as
(11) 
Consider the example depicted in Fig. 6(a). Tree (A) is a derivation tree with initial tree at the root node, and auxiliary trees and in subsequent vertices. The edges are labelled with Gorn addresses of vertices in the auxilliary trees at which adjunctions take place. Performing the adjunctions results in derived tree (B) in Fig. 6(a). The RHS of the resulting model appears at the leaves of the derived tree, and the corresponding model is
(12) 
4.1.2 NARX example
Polynomial NARX models can be described by the equation
(13) 
By restricting auxiliary trees to the set
(14) 
we can restrict the proposed grammar to generate polynomial NARX models only. Consider the example derivation tree (A) in Fig. 6(b), which is an extension of the previous example. The derivation tree consists of the initial tree , and auxiliary trees and . Performing the adjunctions described by the derivation tree results in the derived tree (B) in Fig. 6(b). The corresponding symbolic model is
(15) 
4.1.3 NARMAX example
This example builds on the previous example by using the complete auxiliary tree set and adjoining trees and to the tree . The new derivation tree and derived tree are depicted in Fig. 6(c). The corresponding model,
(16) 
is a polynomial NARMAX model.
4.2 Nonlinear BoxJenkins Extension
Just like the proposed grammar can be scaled down to generate specific dynamic subclasses, it can also be extended to generate models that belong to a more generalized class of models. We illustrate this by extending the proposed grammar to a more generalized models structure  Nonlinear Box Jenkins (NBJ).
In the case of linear systems, a BoxJenkins model structure is an extension of the Output Error (OE) model structure, where the error is modelled as an ARMA process (Ljung, 1999). The BJ class also includes, as special cases, other linear model structures such as ARMAX and OE. In the same spirit, NBJ model structure can be expressed as a Nonlinear Output Error (NOE) model where the error is subsequently modelled as a NARMA process. The NBJ model structure is given by the following equations
(17) 
where and are polynomial functions in terms of their arguments. Notice that the RHS expressions of the equations describing the process and noise dynamics have the same structure that was studied in Sec. 3.2 for NARMAX models (see (8)). Hence, the proposed TAG can be extended to generate NBJ models. Fig. 8 depicts the initial and auxiliary trees of the grammar for NBJ model structures . The structure of the initial tree ensures that all elements in contain two expressions, separated by a comma, that represent the functions and respectively. Each of these expressions can be expanded by adjoining auxiliary trees that ensure that the polynomial structure is maintained.
5 Conclusions
We presented a TAG based concept of a model set, that is more general than that commonly used in the system identification literature. A TAG was proposed that captures the dynamical structure of polynomial NARMAX models. It was demonstrated that subclasses of the polynomial NARMAX class can be represented by choosing an appropriate subset of the elementary trees of . Similarly, more flexible model classes like Nonlinear BoxJenkins can be represented by extending the set of elementary trees. This illustrates that a compact set of elementary trees can be used to express the dynamical relationships across a variety of model classes, thereby enabling the design of TAGbased EA approaches for SI that require minimal userinteraction. The practical soundness of this concept has been demonstrated in Khandelwal et al. (2019), where a TAGbased EA approach was used to identify a nonlinear benchmark dataset with minimal userinteraction, and also in Khandelwal et al. (2019), where the same TAGbased EA approach is used to identify multiple real physical systems and benchmark data set with minimal changes in the methodology itself.
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