Semirings and Their Applications Semirings and Their Applications

Semirings and their Applications Semirings and their Applications

Jonathan S. Golan University ofHaifa. Haifa. Israel

Springer-Science+Business Media, B.V. A c.I.P. Catalogue record for this book is available from the Library of Congress.

ISBN 978-90-481-5252-0 ISBN 978-94-015-9333-5 (eBook) DOI 10.1007/978-94-015-9333-5

Printed on acid-free paper

This book is an updated and considerably expended version of the book "The Theory of Semirings, with Applications to Mathematics and Theoretical science", published by Longinov in 1992.

All Rights Reserved © 1999 Springer Science+Business Media Dordrecht Originally published by Kluwer Academic Publishers in 1999. Softcover reprint ofthe hardcover 1st edition 1999 No part of the material protected by this copyright notice may be reproduced or utilized in any form or by any means, electronic or mechanical, including photocopying, recording or by any information storage and retrieval system, without written permission from the copyright owner. In memory of my mother, Prof. Naomi Golan CONTENTS

Preface IX

1. Hemirings and semirings: definitions and examples 1 2. Sets and relations with values in a semiring 19 3. Building new semirings from old 27 4. Some conditions on semirings . . . 43 5. Complemented elements in semirings 59 6. Ideals in semirings ... 65 7. Prime and semiprime ideals in semirings . .. 85 8. Factor semirings .. . 95 9. Morphisms of semirings 105 10. Kernels of morphisms 121 11. Semirings of fractions 129 12. Euclidean semirings 135 13. Additively-regular semirings 143 14. Semimodules over semirings 149 15. Factor semimodules .. . 163 16. Some constructions for semimodules ... 181 17. Free, proj ective, and injective semimodules 191 18. Localization of semimodules 203 19. Linear algebra over a semiring 211 20. Partially-ordered semirings 223 21. Lattice-ordered semirings 239 22. Complete semirings .. . 247 23. Complete semimodules . .. 259 24. CLO-semirings 267 25. Fixed points of affine maps 285

References ... 307 Index of applications 355 Index of terminology 357 PREFACE

There is no branch of mathematics, however abstract, which may not some day be applied to phenomena of the real world. - Nikolai Ivanovich Lobatchevsky

This book is an extensively-revised and expanded version of "The Theory of Semirings, with Applications in Mathematics and Theoretical Computer Science" [Golan, 1992], first published by Longman. When that book went out of print, it became clear - in light of the significant advances in semi ring theory over th e past years and its new important applications in such areas as idempotent analysis and the theory of discrete-event dynamical systems - that a second edition incorporating minor changes would not be sufficient and that a major revision of the book was in order. Therefore, though the structure of the first «dition was preserved, the text was extensively rewritten and substantially expanded. In particular, references to many interesting and applications of semi ring theory, developed in the past few years , had to be added. Unfortunately, I find that it is best not to go into these applications in detail, for that would entail long digr essions into various domains of pure and applied mathematics which would only detract from the unity of the volume and increase its length considerably. However, I have tried to provide an extensive collection of examples to arouse the reader's interest in applications, as well as sufficient citations to allow the interested reader to locate th em . For the reader's convenience, an index to these citations is given at the end of the book . Thanks are due to the many people who, in the past six years , have offered suggestions and criticisms of th e preceding volume. Foremost among them is Dr. Susan LaGrassa, who was kind enough to send me a detailed list of errors - typo graphical and mathematical - which she found in it . I have tried to correct them all. During the 1997/8 academic year I conducted a seminar on semirings while on sabbatical at the University of Idaho. Many thanks are due to the participants of that seminar, and in particular to Prof. Erol Barbut and Prof. Willy Brandal, for their incisive comments. Prof. Dan Butnariu of th e University of Haifa was also

IX x ______PREFACE _ very instrumental in introducing me to applications of semirings in the th eory of fuzzy sets, as where depar tment al guests Prof. Ivan Chajda, Prof. E. P. Klement and Prof. Radko Mesiar , while Dr. Larry Man evitz of the Department of Com puter Science at th e University of Haifa was always ready to help me understand applica tions in art ificial intelligence and ot her areas of comp uter science . I also owe a large debt to my two former Ph.D. st ude nts, Dr . Wan g Huaxiong and Dr. Wu Fuming, who list ened patiently to my various p-baked ideas as they formed and cont ributed many original insights on semiring theory, which have been incorporated in this edition.

••••

Semirings abound in th e math ematical world aroun d us. Indeed , th e first mathe mati cal structure we encounter - th e set of natural numbers - is a semiring . Other semirings arise naturally in such diverse areas of mathematics as combinatorics, fun ctional analysis, topology, gra ph th eory , Euclidean geometry, probability th e ory, commut ative and non commutative ring th eory, optimizat ion th eory, discr ete event dyn amical systems, auto mata th eory, form al language theory and th e math ematical modeling of quantu m physics and parallel computatio n syste ms. From an algebraic point of view, semirings provide the most natural common generalization of the theories of rin gs and boun ded distributive lattices, and the techniques used in analysing th em are taken from both areas. Histori cally , semirings first appear implicitly in [Dedekind , 1894] and later in [Macaul ay, 1916], [Krull , 1924], [Noether ,1927], and [Lorenzen, 1939] in connection with the st udy of ideals of a ring. T hey also appear in [Hilbert , 1899] and [Hunt ington , 1902] in connection wit h the axiomat ization of the uatural numb ers and nonn egati ve rational numb ers. Semirings per se were first considered explicitly in [Vandiver,"1934], also in connect ion with the axiomatization of the arit hmetic of the natural numbers. His ap proach was lat er develope d in a series of exposito ry art i cles culminating in [Vandiver & Weaver, 1956]. Over the yea rs, semirings have been st udied by various resear chers either in their own right, in an attempt to broaden techniques coming from semigro up theory or ring theory, or in connection with applications. However, despi te such categorical pronoun cement s as "... th e above shows th at th e ring is not th e fu ndamental syste m for associative algebra of dou ble com position" (italics in th e original) found in [Vandiver, 1939], semirings never became popular and the int erest in th em among algebraists gradually petered out, alt hough it never died completely. The only attem pt to present th e algebraic th eory of semirings as an int egral part of modern algebra seems to be in [Redei, 1967] and [Almeida Cost a, 1974]. Nonetheless, semirings - and semimo dules over th em - have becom e an important tool in applied mathemati cs and theoret ical computer science and appear , under vari ous nam es, with consiste nt and increasing frequency in th e literature of those subjects. Were th ere more communication between theoretical algebraists and these utilizers of algebra, it is likely that the former would find in the work of th e latter sufficient ly many "naturally arising" problems to revive and revitalize research in semiring theory in its own right, while the lat ter would find at their disposal a supp ly of th eoretical results which they can use. ______PREFACE _ Xl

Since th e results on semirings are scattered through the mathematical literature and are for most part inaccessible, they are not easily available to those who have to use them.A further problem is that the terminology used by different authors is not st andard many authors use the term "semiring" for wha t we call here a "hemiring" and vice versa. Others, translating directly from the German, use the term "halfring" . Some do not require that a semi ring have a multiplicative identity or even an additive zero . On th e oth er hand, some insist that multiplication , as well as addition, be commutative. In [Gondran & Minoux, 1984], the term "dioid" (i.e., doubl e monoid) is used in place of "semiring" and th e term "semiring" is used in a stronger sense, while [Shier, 1973] prefers th e term "binoid" for a commutative semiring in which addition is idempotent ; others use "dioid" for that purpose. A categorical definition of "semiring" (namely as a semiadditive category having one object) is given in [Manes, 1976]. To add to th e confusion, some sources, e.g. [Sturm, 1986], use the term "semiring" to mean something else entirely. The read er must th erefore be extr emely wary.

• • • •

The notation used throughout the book will be explained as it is introduced. In addition, we will use the following standard notation: lffi = { O,I }, IJ!' = th e set of all positive integers, N = th e set of all nonnegative int egers , Z:: = the set of all integers, Q = t he set of all rational numbers, Q+ = th e set of all nonn egative rational numbers IT = th e unit int erval on th e real line, IR = th e set of all real numbers, IR + = th e set of all nonnegative real numbers, C = th e set of all compl ex numbers. If n is a positive integer th en: Sn is th e group of all permutations of {I , . . . , n}, A n is th e group of all even permutations of {I, ... , n} . If A and B are sets th en: sub(A) is th e family of all subsets of A, fsub(A) is th e family of all finite subsets of A, B A is the set of all functions from A to B.