- Open Access
Analysis approach to finite monoids
© Çevik et al.; licensee Springer. 2013
- Received: 19 November 2012
- Accepted: 9 January 2013
- Published: 22 January 2013
In a previous paper by the authors, a new approach between algebra and analysis has been recently developed. In detail, it has been generally described how one can express some algebraic properties in terms of special generating functions. To continue the study of this approach, in here, we state and prove that the presentation which has the minimal number of generators of the split extension of two finite monogenic monoids has different sets of generating functions (such that the number of these functions is equal to the number of generators) that represent the exponent sums of the generating pictures of this presentation. This study can be thought of as a mixture of pure analysis, topology and geometry within the purposes of this journal.
AMS Subject Classification:11B68, 11S40, 12D10, 20M05, 20M50, 26C05, 26C10.
- p-Cockcroft property
- split extension
- generating functions
- Stirling numbers
- array polynomials
Associated with any (connected) topological space X is its fundamental group or 2-complex (Squier complex) . This can often be specified by means of a presentation. A presentation of a group G or monoid M consists of a set of generators of G or M, together with a collection of relations amongst these generators, such that any other relation amongst the generators is derivable (in a precise sense) from the given relations. Algebraic information about or can be used to obtain topological information about X (cf. ). Many techniques of this branch of mathematics are purely algebraic, and it is possible to achieve much using these techniques. However, in recent years many techniques involving geometric ideas have emerged and are proving more fruitful. These geometric techniques involve graph theory, the theory of tessellations of various surfaces and covering space theory, to name a few.
The number of vertex-colorings of a graph is given by a polynomial on the number of used colors (see ). Based on this polynomial, one can define the chromatic number as the minimum number of colors such that the chromatic polynomial is positive. Recently, our attention has been drawn to the paper  which is a generalization on the chromatic polynomial of a graph subdivision, and basically the authors determine the chromatic number for a simple graph and then present the generalized polynomial for a particular case of graph subdivision. In this reference, the main idea was to express some graph theoretical parameters in terms of special functions. In a similar manner within algebra, by considering a group or a monoid presentation , an approximation from algebra to analysis has been recently developed . To do that, the authors supposed satisfies the special algebraic properties either efficiency or inefficiency while it is minimal. (The reason for choosing efficiency or (minimal) inefficiency was to have an advantage to work on a minimal number of generators.) Then it was investigated whether some generating functions can be applied, and then it was studied what kind of new properties can be obtained by considering special generating functions over . In fact, to investigate this theory, has been taken as the presentation of the split extension and , respectively. Since the results in  imply a new studying area for graphs in the meaning of representation of parameters by generating functions, the results in  will be also given an opportunity to make a new classification of infinite groups and monoids by using generating functions.
This paper can be thought of as another version of . Our general aim here is to define some generating functions in terms of the minimality of the given presentation. This will imply that the minimal number of generators can be represented as generating functions. Similarly as in , our approximation will be applied by considering the split extension. Here, the split extension will be defined as a semi-direct product of two finite monogenic (cyclic) monoids (we may refer to  for details on these monoids). It is obvious that the split extension of two finite structures will also be finite. So, the main difference between the results in here and in the paper  lies in this fact. Because, while a classification over special cases of infinite groups or monoids was given in , the classification in the present paper only focuses on the finite monoids. It is well known that giving some different approximations over finite cases is also as important as giving those over infinite cases.
In the following first subsection, as supportive material, some algebraic facts over split extensions (equivalently, semi-direct products), presentations of finite monogenic monoids, a trivializer set of these presentations and efficiency (equivalently, p-Cockcroft property) are reminded. In Section 2, we present the main material of this paper as two separate subsections. In the first subsection, we present some known results about necessary conditions for the presentation, say , of the split extension of two finite monogenic monoids to be p-Cockcroft (see Proposition 2.1 below) and to be minimal but inefficient (see Proposition 2.3 below). In the final subsection, as a result of all theories until there, we introduce generating functions related to our title (see Theorems 2.5, 2.7 and 2.12 below). In Section 3, by considering one of the functions defined in the previous section, we study this function in the meaning of again generating functions and functional equations (see Theorems 3.1 and 3.3 below).
1.1 Fundamentals of the algebraic part
This subsection should be completely thought of as a part of the expressions in the beginning of this paper.
Let be a monoid presentation where a typical element has the form . Here , are words on X (that is, elements of the free monoid on X). The monoid defined by is the quotient of by the smallest congruence generated by r.
We have a (Squier) graph associated with , where the vertices are the elements of and the edges are the 4-tuples , where , and . The initial, terminal and inversion functions for an edge e as given above are defined by , and .
Two paths π and in a 2-complex are equivalent if there is a finite sequence of paths , where for , the path is obtained from either by inserting or deleting a pair of inverse edges or else by inserting or deleting a defining path for one of the 2-cells of the complex. There is an equivalence relation, ∼, on paths in Γ which is generated by for any edges and of Γ. This corresponds to requiring the closed paths at the vertex to be the defining paths for the 2-cells of a 2-complex having Γ as its 1-skeleton. This 2-complex is called the Squier complex of and denoted by (see, for example, [6–9]). The paths in can be represented by geometric configurations, called monoid pictures. We assume here that the reader is familiar with monoid pictures (see [, Section 4], [, Section 1] or [, Section 2]). Typically, we will use blackboard bold, e.g., , , ℂ, ℙ, as notation for monoid pictures. Atomic monoid pictures are pictures which correspond to paths of length 1. Write for the atomic picture which corresponds to the edge of the Squier complex. Whenever we can concatenate two paths π and in Γ to form the path , then we can concatenate the corresponding monoid pictures ℙ and to form a monoid picture corresponding to . The equivalence of paths in the Squier complex corresponds to an equivalence of monoid pictures. That is, two monoid pictures ℙ and are equivalent if there is a finite sequence of monoid pictures where, for , the monoid picture is obtained from the picture either by inserting or deleting a subpicture , where is an atomic monoid picture, or else by replacing a subpicture by or vice versa, where and are atomic monoid pictures.
A monoid picture is called a spherical monoid picture when the corresponding path in the Squier complex is a closed path. Suppose Y is a collection of spherical monoid pictures over . Two monoid pictures ℙ and are equivalent relative to Y if there is a finite sequence of monoid pictures where, for , the monoid picture is obtained from the picture either by the insertion, deletion and replacement operations of the previous paragraph or else by inserting or deleting a subpicture of the form or of the form , where and . By definition, a set Y of spherical monoid pictures over is a trivializer of if every spherical monoid picture is equivalent to an empty picture relative to Y. By [, Theorem 5.1], if Y is a trivializer for the Squier complex, then the elements of Y generate the first homology group of the Squier complex. The trivializer is also called a set of generating pictures. Some examples and more details of the trivializer can be found in [7–14].
For any monoid picture ℙ over and for any , denotes the exponent sum of R in ℙ which is the number of positive discs labeled by , minus the number of negative discs labeled by . For a non-negative integer n, is said to be n-Cockcroft if (modn), (where congruence (mod0) is taken to be equality) for all and for all spherical pictures ℙ over . Then a monoid ℳ is said to be n-Cockcroft if it admits an n-Cockcroft presentation. In fact, to verify the n-Cockcroft property, it is enough to check for pictures , where Y is a trivializer (see [7, 8]). The 0-Cockcroft property is usually just called Cockcroft. In general, we take n to be equal to 0 or a prime p. Examples of monoid presentations with Cockcroft and p-Cockcroft properties can be found in .
Suppose that is a finite presentation for a monoid ℳ. Then the Euler characteristic is defined by and is defined by . In unpublished work, Pride has shown that . With this background, we define the finite monoid presentation to be efficient if , and we define the monoid ℳ to be efficient if it has an efficient presentation. Moreover, a presentation for ℳ is called minimal if for all presentations of ℳ. There is also interest in finding inefficient finitely presented monoids since if we can find a minimal presentation for a monoid ℳ such that is not efficient, then we have for all presentations defining the same monoid ℳ. Thus, there is no efficient presentation for ℳ, that is, ℳ is not an efficient monoid.
Theorem 1.1 Let be a monoid presentation. Then is efficient if and only if it is p-Cockcroft for some prime p.
Let be a two-sided ideal of generated by the elements , where ℙ is a spherical monoid picture and . Then this ideal is called the second Fox ideal of . More specifically, for a trivializer Y of , the set is generated (as two-sided ideal) by the elements , where and . We note that all this above material given by the consideration ‘left’ can also be applied to ‘right’ for a monoid ℳ.
is a standard monoid presentation for the semi-direct product ℳ.
In , a finite trivializer set has been constructed for the standard presentation , as given in (3), for the semi-direct product ℳ. We will essentially follow  in describing this trivializer set using spherical pictures and certain non-spherical subpictures of these.
If is a positive word on Y, then for any , we denote the word by . If is a positive word on X, then for any , we denote the word by , and this can be represented by a monoid picture, say , as in Figure 2(b). For and the relation in the relation set r, we have two important special cases, and , of this consideration. We should note that these non-spherical pictures consist of only -discs (). Let and . Since , there is a non-spherical picture, say , over with and . Further, let be a relation and . Since θ is a homomorphism, by the definition on , we have that and must represent the same element of the monoid K. That is, . Hence, there is a non-spherical picture over which we denote by with and . In fact, there may be many different ways to construct the pictures and . These pictures must exist, but they are not unique. On the other hand, the picture will depend upon our choices for words , but this is unique once these choices are made.
After all, for , , and , one can construct spherical monoid pictures, say and , by using the non-spherical pictures , , and (see Figures 2, 3 and 4 for the examples of these pictures). Let and be trivializer sets of and , respectively. Also, let and . Then, by [10, 14], it is known that for a presentation , as in (3), a trivializer set of is .
In fact, this is the main section of the paper and it will be given as two subsections under the names of Part I and Part II. Since we will define generating functions by considering the exponent sums of the generating pictures over the presentation of this semi-direct product, the first subsection is aimed to define these generating pictures and the related results about them.
2.1 Part I: generating pictures
In this subsection, we will mainly present the efficiency (equivalently, p-Cockcroft property for a prime p by Theorem 1.1) for the semi-direct products of finite cyclic monoids.
In the rest of the paper, we will assume that the equality in Equation (6) holds when we talk about the semi-direct product M of K by A.
The following result states necessary and sufficient conditions for the presentation of the split extension of two finite monogenic monoids to be efficient.
Proposition 2.1 ()
Remark 2.2 To be an example of Proposition 2.1, one can take
, , , or , , , while and to get 2-Cockcroft property for the presentation in (7), or more generally
for any prime p, , , and to get p-Cockcroft property for the presentation in (7).
In particular, if we choose or 2, then will be inefficient.
Recall that, by the meaning of finite cyclic monoids, cannot be equal to 0. We also note that a similar proof for the following result about minimal but inefficiency of can be found in .
Proposition 2.3 Let M be the semi-direct product of K by A, and let , as in (7), be the presentation for M where and , . If and the subtraction is not even and not equal to 1, then is minimal but inefficient.
Remark 2.4 To be an example of Proposition 2.3, we can consider the following:
For an odd positive integer t, one can take , , , and in the presentation given in (7). Since (), the presentation is minimal but inefficient.
For all such that , one can take , , , and in the presentation given in (7). Since (), the presentation is minimal while it is inefficient.
2.2 Part II: generating functions
By considering the pictures defined in the previous section and also the evaluations obtained from them, we will define the related generating functions. In another words, by taking into account Propositions 2.1 and 2.3, we will reach our main aim over monoids of this paper.
We firstly recall that, as noted in [, Remark 1.1], if a monoid presentation satisfies efficiency or inefficiency (while it is minimal), then it always has a minimal number of generators. Working with the minimal number of elements gives a great opportunity to define related generating functions over this presentation. This will be one of the key points in our results.
Since the coefficients of array polynomials are integers, they find very large application area, especially in system control (cf. ). In fact, these integer coefficients give us the opportunity to use these polynomials in our case. We should note that there also exist some other polynomials, namely Dickson, Bell, Abel, Mittag-Leffler etc., which have integer coefficients which will not be handled in this paper.
From (5), we know that , where . Hence, by considering the meaning and conditions of Proposition 2.1, we obtain the following theorem as one of the main results of this paper.
where and are defined as in (8).
Proof Let us consider the generating pictures , (in Figure 4) with their non-spherical subpictures defined in Figures 2 and 3, and the generating pictures of finite monogenic monoids defined in Figure 1. Recall that by counting the exponent sums of the discs R, S and in the related pictures, the conditions of Proposition 2.1 have been obtained . (For more similar results and applications, one can see the papers [10, 11].)
Then, by reformulating the elements in (10) and (11) of the second Fox ideal , we arrive at the functions in (9) as desired. □
Considering Remark 2.2, we obtain the following corollary as a consequence of Theorem 2.5.
In Proposition 2.3, the minimality (while satisfying inefficiency) of the presentation was expressed in (7). Thus, by considering the meaning and conditions of Proposition 2.3, we obtain the following theorem as another main result of this paper. Since the proof is quite similar to the proof of Theorem 2.5, we omit it.
where is an odd integer and and are defined as in (8).
By considering Remark 2.4, we can have the following consequences of Theorem 2.7.
Remark 2.10 Since both presentations in Propositions 2.1 and 2.3 have the minimal number of generators because of their efficiency or inefficiency (but minimal) status, this situation affected very positively the number and type of generating functions defined on them.
At this point, we should note that for , , , generalized array type polynomials related to the non-negative real parameters have been recently developed (in ) and some elementary properties including recurrence relations of these polynomials have been derived. In fact, by setting and , Equation (8) is obtained.
Remark 2.11 For a future project, one can study the generalization of Theorems 2.5 and 2.7 by using .
where denotes the Kronecker symbol (see [27, 28]). It is known that Stirling numbers are used in combinatorics, in number theory, in discrete probability distributions for finding higher-order moments, etc. We finally note that since is the number of ways to partition a set of n objects into k groups, these numbers find an application area in combinatorics and in theory of partitions.
After that, as a different version of Theorem 2.5 (and so Theorem 2.7), we present the following result.
Theorem 2.12 The efficient presentation in (7) has a set of generating functions in terms of Stirling numbers as given in (13). By taking and is an odd positive integer, we get a set of generating functions in terms of Stirling numbers for the inefficient but minimal presentation of the form as defined in (7).
Hence, we can present the following note.
Remark 2.13 It is clearly seen that Stirling numbers have been only considered in Theorems 2.5 and 2.7 (and the corollaries about them). However, one can also study the γ-Stirling numbers defined in (16) and generalized γ-Stirling numbers defined in (15) to obtain different types of generating functions.
All authors are partially supported by Research Project Offices of Uludağ, Selçuk and Akdeniz Universities, and TUBITAK (The Scientific and Technological Research Council of Turkey).
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