This paper is an introductory, algebraic study of semigroups.

This thesis describes semigroups and the properties of both regular and inverse semigroups.

Thesis (Ph. D.)--University of Wisconsin--Madison,1984. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 182-184).

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The purpose of this paper is to present some fundamental properties of algebraic semigroups. The development of the theory of semigroups has appeared for the most part in the past few years of this century. A semigroup is the result of a weakening of the axioms for a group. Thus all groups are semigroups. That the study of semigroups is very closely related to the abstract study of general transformations is, perhaps, one of the reasons for the rapid development of semigroup theory.

This paper is an algebraic study of selected properties of semigroups. Since a semigroup is a result of weakening the group axioms, all groups are semigroups. One facet of the paper is to demonstrate various semigroup properties that induce the group axioms.

Neste trabalho de dissertação de mestrado, fazemos algumas observações a respeito de dois¶ artigos de pesquisa recentes, os quais estabelecem resultados falsos sobre existência de soluções controlabilidade exata de sistemas de controle abstratos do tipo neutro modelados na forma¶8<:¶ d ¶ dt ¶(x(t) + G(t, xt)) = Ax(t) + F(t, xt) + Cu(t), t 2 I = (0, a],¶ x0 = ',¶ (4)¶ onde A é o gerador infinitesimal de um C0-semigrupo de operadores lineares definidos sobre um espaço de Banach X; a função xt representa a história do estado no tempo t; C : U ! X ´e um¶ perador linear limitado, U é um espaço de Banach e F,G são funções apropriadas.¶ Motivados pelo anterior, neste trabalho estudamos a existência de soluções fracas para o sistema ¶ abstrato 8<:¶ d¶ dt¶ [(I &#8722; B)x(t)] = Ax(t) + F(t, x(t)), t 2 I = (0, a],¶ x(0) = x0 2 X,¶ (5)¶ onde B : X ! X é um operador linear limitado. Além do anterior, introduzimos e estudamos a “¶ controlabilidade [D(A)]-aproximada " para o sistema de controle¶ 8<:¶ d¶ dt¶ [(I &#8722; B)x(t)] = Ax(t) + Cu(t) + f(t, x(t)), t 2 I = (0, a],¶ x(0) = x0 2 X.¶ (6)¶. Observamos que nossos comentários relacionados com a controlabilidade exata de sistemas¶ neutros, foram publicados recentemente em journal Computers & Mathematics with Applications,¶ veja [12] para detalhes. Também mencionamos que no pre-print [16] são resumidos nossos resultados¶ relacionados a existência de soluções fracas de (5) e a controlabilidade do sistema de controle (6). / In this work, we report two technical errors in some recent papers treating on existence and¶ xact controllability of solutions for a class of partial neutral functional differential control systems¶ described in the abstract form¶ 8<:¶ d¶ dt¶ (x(t) + G(t, xt)) = Ax(t) + F(t, xt) + Cu(t), t 2 I = (0, a],¶ x0 = '.¶ (1)¶ In this system A is the infinitesimal generator of an C0-semigroup of bounded linear operators¶ defined on a Banach space X; the functions xt are the histories, C : U ! X is a bounded operator,¶ U is a Banach space and and F,G are appropriate functions.¶ Additionally, by using some simple perturbation criterion, we discuss the existence of mild¶ solution for the system¶ 8<:¶ d¶ dt¶ [x(t) + Bx(t)] = Ax(t) + f(t, x(t)), t 2 I = [0, a],¶ x(0) = x0 2 X,¶ (2)¶ where B is a bounded linear operator. We also introduce the concept of approximate [D(A)]-¶ controlability and establish conditions under which the associated control system¶ 8<:¶ d¶ dt¶ [x(t) + Bx(t)] = Ax(t) + Cu(t) + f(t, x(t)), t 2 I = [0, a],¶ x(0) = x0 2 X,¶ (3)¶ is approximate [D(A)]- controllable.¶ We mention that our observation on exact controllability of abstract neutral system was recently¶ published in journal Computers & Mathematics with Applications, see [12] for details. We also note,¶ that the pre-print [16] contain some new results concerning existence of solutions and controllability¶ for the systems (2) and (3) respectively.

by Kam Siu Man. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1991. / Includes bibliographical references. / Chapter Chapter I --- On a generalized Kuratowski's closure theorem / Chapter §0. --- Introduction --- p.1 / Chapter §1. --- Counter examples and the idea of migration --- p.1 / Chapter §2. --- The use of immigration regulations --- p.11 / Chapter §3. --- Remarks on the boundary function and the complementary function --- p.35 / Chapter Chapter II --- A problem in number theory / Chapter §1. --- Introduction --- p.37 / Chapter §2. --- Preliminaries --- p.37 / Chapter §3. --- Technical Lemmas --- p.41 / Chapter §4. --- Main Theorems --- p.45 / Chapter §5. --- Closing Remarks --- p.49 / Chapter Chapter III --- Stiff elements in semigroups and their expansions / Chapter §1. --- Introduction --- p.51 / Chapter §2. --- String expansions of semigroups --- p.51 / Chapter §3. --- Stiff Semigroups --- p.59 / Chapter §4. --- Construction of examples --- p.71 / Chapter §5. --- Partition of semigroups by stiffness --- p.74