Covers for Regular Semigroups and an Application to Complexity
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Symplectic Group Actions and Covering Spaces
Symplectic group actions and covering spaces Montaldi, James and Ortega, Juan-Pablo 2009 MIMS EPrint: 2007.84 Manchester Institute for Mathematical Sciences School of Mathematics The University of Manchester Reports available from: http://eprints.maths.manchester.ac.uk/ And by contacting: The MIMS Secretary School of Mathematics The University of Manchester Manchester, M13 9PL, UK ISSN 1749-9097 Symplectic Group Actions and Covering Spaces James Montaldi & Juan-Pablo Ortega January 2009 Abstract For symplectic group actions which are not Hamiltonian there are two ways to define reduction. Firstly using the cylinder-valued momentum map and secondly lifting the action to any Hamiltonian cover (such as the universal cover), and then performing symplectic reduction in the usual way. We show that provided the action is free and proper, and the Hamiltonian holonomy associated to the action is closed, the natural projection from the latter to the former is a symplectic cover. At the same time we give a classification of all Hamiltonian covers of a given symplectic group action. The main properties of the lifting of a group action to a cover are studied. Keywords: lifted group action, symplectic reduction, universal cover, Hamiltonian holonomy, momentum map MSC2000: 53D20, 37J15. Introduction There are many instances of symplectic group actions which are not Hamiltonian—ie, for which there is no momentum map. These can occur both in applications [13] as well as in fundamental studies of symplectic geometry [1, 2, 5]. In such cases it is possible to define a “cylinder valued momentum map” [3], and then perform symplectic reduction with respect to this map [16, 17]. -
1 the Spin Homomorphism SL2(C) → SO1,3(R) a Summary from Multiple Sources Written by Y
1 The spin homomorphism SL2(C) ! SO1;3(R) A summary from multiple sources written by Y. Feng and Katherine E. Stange Abstract We will discuss the spin homomorphism SL2(C) ! SO1;3(R) in three manners. Firstly we interpret SL2(C) as acting on the Minkowski 1;3 spacetime R ; secondly by viewing the quadratic form as a twisted 1 1 P × P ; and finally using Clifford groups. 1.1 Introduction The spin homomorphism SL2(C) ! SO1;3(R) is a homomorphism of classical matrix Lie groups. The lefthand group con- sists of 2 × 2 complex matrices with determinant 1. The righthand group consists of 4 × 4 real matrices with determinant 1 which preserve some fixed real quadratic form Q of signature (1; 3). This map is alternately called the spinor map and variations. The image of this map is the identity component + of SO1;3(R), denoted SO1;3(R). The kernel is {±Ig. Therefore, we obtain an isomorphism + PSL2(C) = SL2(C)= ± I ' SO1;3(R): This is one of a family of isomorphisms of Lie groups called exceptional iso- morphisms. In Section 1.3, we give the spin homomorphism explicitly, al- though these formulae are unenlightening by themselves. In Section 1.4 we describe O1;3(R) in greater detail as the group of Lorentz transformations. This document describes this homomorphism from three distinct per- spectives. The first is very concrete, and constructs, using the language of Minkowski space, Lorentz transformations and Hermitian matrices, an ex- 4 plicit action of SL2(C) on R preserving Q (Section 1.5). -
UNIT-REGULAR ORTHODOX SEMIGROUPS by R
UNIT-REGULAR ORTHODOX SEMIGROUPS by R. B. McFADDEN (Received 5 May, 1983) Introduction. Unit-regular rings were introduced by Ehrlich [4]. They arose in the search for conditions on a regular ring that are weaker than the ACC, DCC, or finite Goldie dimension, which with von Neumann regularity imply semisimplicity. An account of unit-regular rings, together with a good bibliography, is given by Goodearl [5]. The basic definition of unit-regularity is purely multiplicative; it is simply that for each element x of a monoid S (initially a ring R with identity) there is a unit u of S for which x = xux. The concept of a unit-regular semigroup is a natural one; for example, the full transformation semigroup on a finite set, and the semigroup of endomorphisms of a finite-dimensional vector space, are unit-regular semigroups [1]. Unit-regularity has been studied by Chen and Hsieh [2], by Tirasupa [9], and by McAlister [6]. The connection between unit-regularity and finiteness conditions has been considered by D'Alarcao [3]. The problem of describing the structure of an arbitrary unit-regular semigroup S is difficult. It appears reasonable to attempt to provide such a description in terms of the group of units of S and the set of idempotents of S, and in this direction Blyth and McFadden did determine the structure of a narrow class of unit-regular semigroups. Calling a semigroup S uniquely unit orthodox if it is orthodox and, for each x in S, there exists a unique unit u of S for which x = xux, they proved that every such semigroup is a semidirect product of a group (the group of units of S) and a band (the band of idempotents of S). -
MTH 507 Midterm Solutions
MTH 507 Midterm Solutions 1. Let X be a path connected, locally path connected, and semilocally simply connected space. Let H0 and H1 be subgroups of π1(X; x0) (for some x0 2 X) such that H0 ≤ H1. Let pi : XHi ! X (for i = 0; 1) be covering spaces corresponding to the subgroups Hi. Prove that there is a covering space f : XH0 ! XH1 such that p1 ◦ f = p0. Solution. Choose x ; x 2 p−1(x ) so that p :(X ; x ) ! (X; x ) and e0 e1 0 i Hi ei 0 p (X ; x ) = H for i = 0; 1. Since H ≤ H , by the Lifting Criterion, i∗ Hi ei i 0 1 there exists a lift f : XH0 ! XH1 of p0 such that p1 ◦f = p0. It remains to show that f : XH0 ! XH1 is a covering space. Let y 2 XH1 , and let U be a path-connected neighbourhood of x = p1(y) that is evenly covered by both p0 and p1. Let V ⊂ XH1 be −1 −1 the slice of p1 (U) that contains y. Denote the slices of p0 (U) by 0 −1 0 −1 fVz : z 2 p0 (x)g. Let C denote the subcollection fVz : z 2 f (y)g −1 −1 of p0 (U). Every slice Vz is mapped by f into a single slice of p1 (U), −1 0 as these are path-connected. Also, since f(z) 2 p1 (y), f(Vz ) ⊂ V iff f(z) = y. Hence f −1(V ) is the union of the slices in C. 0 −1 0 0 0 Finally, we have that for Vz 2 C,(p1jV ) ◦ (p0jVz ) = fjVz . -
Theory of Covering Spaces
THEORY OF COVERING SPACES BY SAUL LUBKIN Introduction. In this paper I study covering spaces in which the base space is an arbitrary topological space. No use is made of arcs and no assumptions of a global or local nature are required. In consequence, the fundamental group ir(X, Xo) which we obtain fre- quently reflects local properties which are missed by the usual arcwise group iri(X, Xo). We obtain a perfect Galois theory of covering spaces over an arbitrary topological space. One runs into difficulties in the non-locally connected case unless one introduces the concept of space [§l], a common generalization of topological and uniform spaces. The theory of covering spaces (as well as other branches of algebraic topology) is better done in the more general do- main of spaces than in that of topological spaces. The Poincaré (or deck translation) group plays the role in the theory of covering spaces analogous to the role of the Galois group in Galois theory. However, in order to obtain a perfect Galois theory in the non-locally con- nected case one must define the Poincaré filtered group [§§6, 7, 8] of a cover- ing space. In §10 we prove that every filtered group is isomorphic to the Poincaré filtered group of some regular covering space of a connected topo- logical space [Theorem 2]. We use this result to translate topological ques- tions on covering spaces into purely algebraic questions on filtered groups. In consequence we construct examples and counterexamples answering various questions about covering spaces [§11 ]. In §11 we also -
Restriction Semigroups Peter R
Marquette University e-Publications@Marquette Mathematics, Statistics and Computer Science Mathematics, Statistics and Computer Science, Faculty Research and Publications Department of 1-1-2014 Varieties of P-Restriction Semigroups Peter R. Jones Marquette University, [email protected] Accepted version. Communications in Algebra, Vol. 42, No. 4 (2014): 1811-1834. DOI. © 2014 Taylor & Francis. Used with permission. Varieties of P -restriction semigroups Peter R. Jones November 6, 2012 Abstract The restriction semigroups, in both their one-sided and two-sided versions, have arisen in various fashions, meriting study for their own sake. From one historical perspective, as `weakly E-ample' semigroups, the definition revolves around a `designated set' of commuting idempotents, better thought of as projections. This class includes the inverse semigroups in a natural fashion. In a recent paper, the author introduced P -restriction semigroups in order to broaden the notion of `projection' (thereby encompassing the regular ∗-semigroups). That study is continued here from the varietal perspective introduced for restriction semigroups by V. Gould. The relationship between varieties of regular ∗-semigroups and varieties of P - restriction semigroups is studied. In particular, a tight relationship exists between varieties of orthodox ∗-semigroups and varieties of `orthodox' P -restriction semigroups, leading to concrete descriptions of the free orthodox P -restriction semigroups and related structures. Specializing further, new, elementary paths are found for descriptions of the free restriction semigroups, in both the two-sided and one-sided cases. In [13], the author introduced P -restriction semigroups as a common generalization of the restriction semigroups (or `weakly E-ample' semigroups) and the regular ∗-semigroups, defining them as bi-unary semigroups { semigroups with two additional unary operations, + and ∗ { satisfying a set of simple identities. -
MATH 215B MIDTERM SOLUTIONS 1. (A) (6 Marks) Show That a Finitely
MATH 215B MIDTERM SOLUTIONS 1. (a) (6 marks) Show that a finitely generated group has only a finite number of subgroups of a given finite index. (Hint: Do it for a free group first.) (b) (6 marks) Show that an index n subgroup H of a group G has at most n conjugate subgroups gHg−1 in G. Apply this to show that there exists a normal subgroup K ⊂ G of finite index in G with K ⊂ H. Solution (a) Suppose we have a finitely-generated free group ∗nZ, and a subgroup H of finite index k. We can build a finite graph X having fundamental group ∗nZ; it is a wedge of n circles, having 1 vertex and n edges. The subgroup p H determines a connected based covering space Xe −→ X, which is also a graph. The number of sheets is the index of p∗π1(Xe) in π1(X), which is k. So Xe is a based graph with k vertices and nk edges. Up to based isomorphism, there are only finitely many based graphs with k vertices and nk edges. So by the classification of covering maps, there are only finitely many subgroups H of index k. Now suppose we have a finitely-generated group. Every finitely-generated group is a quotient of a finitely-generated free group, so we can assume our group is ∗nZ/K for some normal subgroup K ⊂ ∗nZ. An index k subgroup of ∗nZ/K is of the form H/K where H is an index k subgroup of ∗nZ that contains K. -
Beyond Regular Semigroups
View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by OpenGrey Repository Beyond Regular Semigroups Yan Hui Wang PhD University of York Department of Mathematics March 2012 Abstract The topic of this thesis is the class of weakly U-abundant semigroups. This class is very wide, containing inverse, orthodox, regular, ample, adequate, quasi- adequate, concordant, abundant, restriction, Ehresmann and weakly abundant semigroups. A semigroup S with subset of idempotents U is weakly U-abundant if every RU -class and every LU -class contains an idempotent of U, where RU and LU are relations extending the well known Green’s relations R and L. We assume throughout that our semigroups satisfy a condition known as the Con- gruence Condition (C). We take several approaches to weakly U-abundant semigroups. Our first re- sults describe those that are analogous to completely simple semigroups. Together with an existing result of Ren this determines the structure of those weakly U- abundant semigroups that are analogues of completely regular semigroups, that is, they are superabundant. Our description is in terms of a semilattice of rectan- gular bands of monoids. The second strand is to aim for an extension of the Hall-Yamada theorem for orthodox semigroups as spined products of inverse semigroups and fundamental orthodox semigroups. To this end we consider weakly B-orthodox semigroups, where B is a band. We note that if B is a semilattice then a weakly B-orthodox semigroup is exactly an Ehresmann semigroup. We provide a description of a weakly B-orthodox semigroup S as a spined product of a fundamental weakly B- orthodox semigroup SB (depending only on B) and S/γB, where B is isomorphic to B and γB is the analogue of the least inverse congruence on an orthodox semigroup. -
Heat Kernels on Covering Spaces and Topological Invariants
J. DIFFERENTIAL GEOMETRY 35(1992) 471-510 HEAT KERNELS ON COVERING SPACES AND TOPOLOGICAL INVARIANTS JOHN LOTT I. Introduction It is well known that there are relationships between the heat flow, acting on differential forms on a closed oriented manifold M, and the topology of M. From Hodge theory, one can recover the Betti numbers of M from the heat flow. Furthermore, Ray and Singer [42] defined an analytic tor- sion, a smooth invariant of acyclic flat bundles on M, and conjectured that it equals the classical Reidemeister torsion. This conjecture was proved to be true independently by Cheeger [7] and Muller [40]. The analytic torsion is nonzero only on odd-dimensional manifolds, and behaves in some ways as an odd-dimensional counterpart of the Euler characteristic [20]. If M is not simply-connected, then there is a covering space analog of the Betti numbers. Using the heat flow on the universal cover M, one can define the L2-Betti numbers of M [1] by taking the trace of the heat kernel not in the ordinary sense, but as an element of a certain type II von Neumann algebra. More concretely, this amounts to integrating the local trace of the heat kernel over a fundamental domain in M. In §11, we summarize this theory. We consider the covering space analog of the analytic torsion. This iΛanalytic torsion has the same relation to the iΛcohomology as the ordinary analytic torsion bears to de Rham cohomology. In §111 we define the iΛanalytic torsion <9^(M), under a technical assumption which we discuss later, and prove its basic properties. -
Simple Semirings
International Journal of Engineering Inventions e-ISSN: 2278-7461, p-ISSN: 2319-6491 Volume 2, Issue 7 (May 2013) PP: 16-19 Simple Semirings P. Sreenivasulu Reddy1, Guesh Yfter tela2 Department of mathematics, Samara University, Samara, Afar Region, Ethiopia Post Box No.131 Abstract: Author determine different additive structures of simple semiring which was introduced by Golan [3]. We also proved some results based on the papers of Fitore Abdullahu [1]. I. Introduction This paper reveals the additive structures of simple semirings by considering that the multiplicative semigroup is rectangular band. 1.1. Definition: A semigroup S is called medial if xyzu = xzyu, for every x, y, z, u in S. 1.2. Definition: A semigroup S is called left (right) semimedial if it satisfies the identity x2yz = xyxz (zyx2 = zxyx), where x,y,z S and x, y are idempotent elements. 1.3. Definition: A semigroup S is called a semimedial if it is both left and right semimedial. Example: The semigroup S is given in the table is I-semimedial * a b c a b b b b b b b c c c c 1.4. Definition: A semigroup S is called I- left (right) commutative if it satisfies the identity xyz = yxz (zxy = zyx), where x, y are idempotent elements. 1.5. Definition: A semigroup S is called I-commutative if it satisfies the identity xy = yx, where x,y S and x, y are idempotent elements. Example: The semigroup S is given in the table is I-commutative. * a b c a b b a b b b b c c b c 1.6. -
A System of Bi-Identities for Locally Inverse Semigroups
PROCEEDINGSOF THE AMERICANMATHEMATICAL SOCIETY Volume 123, Number 4, April 1995 A SYSTEM OF BI-IDENTITIES FOR LOCALLYINVERSE SEMIGROUPS K. AUINGER (Communicated by Ronald M. Solomon) Abstract. A class of regular semigroups closed under taking direct products, regular subsemigroups, and homomorphic images is an existence-variety (or e- variety) of regular semigroups. Each e-variety of locally inverse semigroups can be characterized by a set of bi-identities. These are identities of terms of type (2, 2) in two sorts of variables X and X'. In this paper we obtain a basis of bi-identities for the e-variety of locally inverse semigroups and for certain sub-e-varieties. 1. Introduction In order to study regular semigroups from a "universal" perspective, Hall [6, 7, 8] introduced the notion of existence variety (or e-variety) of regular semigroups: this is a class of regular semigroups closed under taking direct products, regular subsemigroups, and homomorphic images. He showed that the collection of all ^-varieties of regular semigroups forms a complete lattice under inclusion (containing the lattices of inverse, respectively completely regular, semigroup varieties as ideal sublattices). Independently, Kadourek and Szendrei [10] introduced the concept of e- variety for the class of orthodox semigroups, using the term "bivariety". In addition they showed that e-varieties of orthodox semigroups are determined by sets of bi-identities. In this context a bi-identity is a semigroup identity in two sorts of variables x and x'. A bi-identity u = v is said to hold in an orthodox semigroup S if each substitution of the variables in u and v by elements of S in such a way that x and x' are substituted by mutually inverse elements of S yields the same element of S on both sides. -
The Concept of Structural Regularity 1
PORTUGALIAE MATHEMATICA Vol. 53 Fasc. 4 { 1996 THE CONCEPT OF STRUCTURAL REGULARITY Samuel J.L. Kopamu Abstract: We introduce the class of structurally regular semigroups. Examples of such semigroups are presented, and relationships with other known generalisations of the class of regular semigroups are explored. Some fundamental results and concepts about regular semigroups are generalised to this new class. In particular, a version of Lallement's Lemma is proved. 1 { Preliminaries and introduction An element x of a semigroup is said to be (von Neumann) regular if there exists an (inverse) element y such that xyx = x and yxy = y; and semigroups consisting entirely of such elements are called regular. Regular semigroups have received wide attention (see for example, [16], [17], and [30]). In the literature, the set of all inverses of a regular element x is denoted by V (x). An element x is said to be an idempotent if x2 = x; and semigroups consisting entirely of idempotent elements are called bands. Inverse semigroups are just the regular semigroups with commuting idempotents, or equivalently, they are regular semigroups with unique inverses. Regular semigroups with a unique idempotent element are easily seen to be groups; semigroups that are unions of groups are called completely regular; and regular semigroups whose idempotent elements form a subsemigroup are called orthodox. The very ¯rst class of semigroups to be studied was the class of groups, and some of the important results in semigroup theory came about as a result of attempting to generalise results from group theory. For example, the Vagner{ Preston Representation Theorem for inverse semigroups was influenced by Cay- Received: December 1, 1995; Revised: February 17, 1996.