DOCSLIB.ORG

Flows in Graphs and Matroids ICGT Grenoble, 2014

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Flows in Graphs and Matroids ICGT Grenoble, 2014 Flows in graphs and matroids ICGT Grenoble, 2014 Bertrand Guenin, University of Waterloo July, 2014 Flows in graphs and matroids ICGT Grenoble, 2014 2 2 2 Demand edges 1 2 3 Capacity edges 1 3 1 Defining flows A flow instance: • Graph G = (V; E) • Demand edges: Σ E(G) ⊆ • Capacity edges: E(G) Σ n • Integer weights w 0 for all e E e ≥ 2 Flows in graphs and matroids ICGT Grenoble, 2014 2 2 2 Demand edges 1 2 3 Capacity edges 1 3 1 Defining flows A flow instance: • Graph G = (V; E) • Demand edges: Σ E(G) ⊆ • Capacity edges: E(G) Σ n • Integer weights w 0 for all e E e ≥ 2 Flows in graphs and matroids ICGT Grenoble, 2014 y 0 for all C C is a flow if C ≥ 2 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e 2 2 2 1 2 3 1 3 1 Let C be the set of circuits containing exactly one demand edge. Flows in graphs and matroids ICGT Grenoble, 2014 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e 2 2 2 1 2 3 1 3 1 Let C be the set of circuits containing exactly one demand edge. y 0 for all C C is a flow if C ≥ 2 Flows in graphs and matroids ICGT Grenoble, 2014 Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e 2 2 2 1 2 3 1 3 1 Let C be the set of circuits containing exactly one demand edge. y 0 for all C C is a flow if C ≥ 2 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Flows in graphs and matroids ICGT Grenoble, 2014 2 2 2 1 2 3 1 3 1 Let C be the set of circuits containing exactly one demand edge. y 0 for all C C is a flow if C ≥ 2 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e Flows in graphs and matroids ICGT Grenoble, 2014 2 2 2 1 2 3 1 3 1 Let C be the set of circuits containing exactly one demand edge. y 0 for all C C is a flow if C ≥ 2 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e Flows in graphs and matroids ICGT Grenoble, 2014 Let C be the set of circuits containing exactly one demand edge. y 0 for all C C is a flow if C ≥ 2 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e 2 yC =1 2 2 1 1 2 3 1 3 1 Flows in graphs and matroids ICGT Grenoble, 2014 Let C be the set of circuits containing exactly one demand edge. y 0 for all C C is a flow if C ≥ 2 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e 2 yC =1 2 2 1 1 2 3 yC2 =1 1 3 1 Flows in graphs and matroids ICGT Grenoble, 2014 Let C be the set of circuits containing exactly one demand edge. y 0 for all C C is a flow if C ≥ 2 Capacity constraints: for every capacity edge e, X (y : e C C ) w C 2 2 ≤ e Demand constraints: for every demand edge e, X (y : e C C )= w C 2 2 e 2 yC =1 2 2 1 1 2 3 yC2 =1 yC =2 1 3 3 1 Flows in graphs and matroids ICGT Grenoble, 2014 NO 2 3 4 1 4 3 A necessary condition Do we always have a flow? Flows in graphs and matroids ICGT Grenoble, 2014 2 3 4 1 4 3 A necessary condition Do we always have a flow? NO Flows in graphs and matroids ICGT Grenoble, 2014 Demand across cut = 2 + 3 Capacity across cut = 4 No flow The cut condition: For every cut: Demand across the cut Capacity across the cut. ≤ A necessary condition Do we always have a flow? NO 2 3 4 1 4 3 Flows in graphs and matroids ICGT Grenoble, 2014 Capacity across cut = 4 No flow The cut condition: For every cut: Demand across the cut Capacity across the cut. ≤ A necessary condition Do we always have a flow? NO 2 Demand across cut = 2 + 3 3 4 1 4 3 Flows in graphs and matroids ICGT Grenoble, 2014 No flow The cut condition: For every cut: Demand across the cut Capacity across the cut. ≤ A necessary condition Do we always have a flow? NO 2 Demand across cut = 2 + 3 Capacity across cut = 4 3 4 1 4 3 Flows in graphs and matroids ICGT Grenoble, 2014 The cut condition: For every cut: Demand across the cut Capacity across the cut. ≤ A necessary condition Do we always have a flow? NO 2 Demand across cut = 2 + 3 Capacity across cut = 4 3 4 1 4 3 No flow Flows in graphs and matroids ICGT Grenoble, 2014 A necessary condition Do we always have a flow? NO 2 Demand across cut = 2 + 3 Capacity across cut = 4 3 4 1 4 3 No flow The cut condition: For every cut: Demand across the cut Capacity across the cut. ≤ Flows in graphs and matroids ICGT Grenoble, 2014 NO All capacities/demands = 1 Integer flow Is the cut condition sufficient for existence of integer flow? Flows in graphs and matroids ICGT Grenoble, 2014 Integer flow Is the cut condition sufficient for existence of integer flow? NO All capacities/demands = 1 Flows in graphs and matroids ICGT Grenoble, 2014 Theorem [Seymour 1977] There exists an integer flow if 1. the cut-condition holds, 2. there is no odd-K4 minor. Integer flow Is the cut condition sufficient for existence of integer flow? NO All capacities/demands = 1 Flows in graphs and matroids ICGT Grenoble, 2014 Integer flow Is the cut condition sufficient for existence of integer flow? NO All capacities/demands = 1 Theorem [Seymour 1977] There exists an integer flow if 1. the cut-condition holds, 2. there is no odd-K4 minor. Flows in graphs and matroids ICGT Grenoble, 2014 Minors for (G; Σ): • Delete e E(G), 2 • Contract e = Σ, 2 • Resign: replace Σ by Σ δ(U) where δ(U) is a cut. 4 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), 2 • Contract e = Σ, 2 • Resign: replace Σ by Σ δ(U) where δ(U) is a cut. 4 Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), • Contract2e = Σ, • Resign: replace2 Σ by Σ δ(U) where δ(U) is a cut. 4 Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), • Contract2e = Σ, • Resign: replace2 Σ by Σ δ(U) where δ(U) is a cut. 4 Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), • Contract2e = Σ, • Resign: replace2 Σ by Σ δ(U) where δ(U) is a cut. 4 Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), • Contract2e = Σ, • Resign: replace2 Σ by Σ δ(U) where δ(U) is a cut. 4 Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), • Contract2e = Σ, • Resign: replace2 Σ by Σ δ(U) where δ(U) is a cut. 4 Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), 2 • Contract e = Σ, 2 • Resign: replace Σ by Σ δ(U) where δ(U) is a cut. 4 Flows in graphs and matroids ICGT Grenoble, 2014 A pair (G; Σ) where Σ E(G) is a signed graph. Σ is the signature ⊆ Minors for (G; Σ): • Delete e E(G), 2 • Contract e = Σ, 2 • Resign: replace Σ by Σ δ(U) where δ(U) is a cut.
Load more

Recommended publications
  • Generalization of Pinching Operation to Binary Matroids
    J. Algebra Comb. Discrete Appl. Received: 14 April 2020 7(3) • 247–258 Accepted: 1 May 2020 Journal of Algebra Combinatorics Discrete Structures and Applications Generalization of pinching operation to binary matroids Research Article Vahid Ghorbani, Ghodratollah Azadi, Habib Azanchiler Abstract: In this paper, we generalize the pinching operation on two edges of graphs to binary matroids and investigate some of its basic properties. For n ≥ 2, the matroid that is obtained from an n-connected matroid by this operation is a k-connected matroid with k 2 f2; 3; 4g or is a disconnected matroid. We find conditions to guarantee this k. Moreover, we show that Eulerian binary matroids are char- acterized by this operation and we also provide some interesting applications of this operation. 2010 MSC: 05B35 Keywords: Binary matroid, Connectivity, Pinching, Splitting, Splitting off, Element splitting 1. Introduction The matroid and graph notations and terminology used here will follow [4] and [7]. Let G be a graph p and S be a subset of the edge set E(G). We denote by GS the graph that is obtained from G by adding p a new vertex w and replacing any edge uv 2 S with two edges uw and wv. The transition from G to GS is called the k-pinching operation [2] where k = jSj. If S is the empty set, then 0-pinching means adding a new single vertex w. If jSj = 1, then applying the pinching operation is equivalent to the subdivision p of uv where S = fuvg. If S = E(G), then we have the parallel classes on the new vertex w on GS such that the sequence of the cardinality of them is equal to the degree sequence of the vertices of G.
    [Show full text]
  • Arxiv:1910.05689V1 [Math.CO]
    ON GRAPHIC ELEMENTARY LIFTS OF GRAPHIC MATROIDS GANESH MUNDHE1, Y. M. BORSE2, AND K. V. DALVI3 Abstract. Zaslavsky introduced the concept of lifted-graphic matroid. For binary matroids, a binary elementary lift can be defined in terms of the splitting operation. In this paper, we give a method to get a forbidden-minor characterization for the class of graphic matroids whose all lifted-graphic matroids are also graphic using the splitting operation. Keywords: Elementary lifts; splitting; binary matroids; minors; graphic; lifted-graphic Mathematics Subject Classification: 05B35; 05C50; 05C83 1. Introduction For undefined notions and terminology, we refer to Oxley [13]. A matroid M is quotient of a matroid N if there is a matroid Q such that, for some X ⊂ E(Q), N = Q\X and M = Q/X. If |X| = 1, then M is an elementary quotient of N. A matroid N is a lift of M if M is a quotient of N. If M is an elementary quotient of N, then N is an elementary lift of M. A matroid N is a lifted-graphic matroid if there is a matroid Q with E(Q)= E(N) ∪ e such that Q\e = N and Q/e is graphic. The concept of lifted-graphic matroid was introduced by Zaslavsky [19]. Lifted-graphic matroids play an important role in the matroid minors project of Geelen, Gerards and Whittle [9, 10]. Lifted- graphic matroids are studied in [4, 5, 6, 7, 19]. This class is minor-closed. In [5], it is proved that there exist infinitely many pairwise non-isomorphic excluded minors for the class of lifted-graphic matroids.
    [Show full text]
  • The Circuit and Cocircuit Lattices of a Regular Matroid
    University of Vermont ScholarWorks @ UVM Graduate College Dissertations and Theses Dissertations and Theses 2020 The circuit and cocircuit lattices of a regular matroid Patrick Mullins University of Vermont Follow this and additional works at: https://scholarworks.uvm.edu/graddis Part of the Mathematics Commons Recommended Citation Mullins, Patrick, "The circuit and cocircuit lattices of a regular matroid" (2020). Graduate College Dissertations and Theses. 1234. https://scholarworks.uvm.edu/graddis/1234 This Thesis is brought to you for free and open access by the Dissertations and Theses at ScholarWorks @ UVM. It has been accepted for inclusion in Graduate College Dissertations and Theses by an authorized administrator of ScholarWorks @ UVM. For more information, please contact [email protected]. The Circuit and Cocircuit Lattices of a Regular Matroid A Thesis Presented by Patrick Mullins to The Faculty of the Graduate College of The University of Vermont In Partial Fulfillment of the Requirements for the Degree of Master of Science Specializing in Mathematical Sciences May, 2020 Defense Date: March 25th, 2020 Dissertation Examination Committee: Spencer Backman, Ph.D., Advisor Davis Darais, Ph.D., Chairperson Jonathan Sands, Ph.D. Cynthia J. Forehand, Ph.D., Dean of Graduate College Abstract A matroid abstracts the notions of dependence common to linear algebra, graph theory, and geometry. We show the equivalence of some of the various axiom systems which define a matroid and examine the concepts of matroid minors and duality before moving on to those matroids which can be represented by a matrix over any field, known as regular matroids. Placing an orientation on a regular matroid M allows us to define certain lattices (discrete groups) associated to M.
    [Show full text]
  • Cycles of Binary Matroids with No F* 7 -Minor
    A. Galluccio, M. Loebl ∗ CYCLES OF BINARY MATROIDS WITH NO F7 -MINOR R. 431 Maggio 1996 Anna Galluccio — Istituto di Analisi dei Sistemi ed Informatica del CNR, Viale Manzoni 30, 00185 Roma, Italy Martin Loebl — Department of Applied Mathematics, Charles University, 11800 Prague 1, Czech Republic. Abstract We study the circuit lattice of a binary matroid, i.e. the set of all integer linear combinations of circuits of a binary matroid. In this paper we prove ∗ that if a binary matroid B does not contain an F7 -minor then the circuit lattice of B admits a basis consisting of cycles. 1 Introduction We will show in this paper that the lattice generated by the circuits of a ∗ binary matroid without an F7 -minor has a basis consisting of cycles. It may be viewed as a continuation of our paper [1] where bases of lattices of directed cycles of digraphs and cycles of graphs were described. Let us recall some basic definitions related to binary matroids (see [7] for an exposition of the matroid theory). Let V d denote the d-dimensional d d+1 vector space over GF [2] and F = V −0V d+1 the d-dimensional projective 2 space. F is called Fano plane and denoted sometimes F7. Simple binary matroids are the nonempty subsets of a projective space. Each binary matroid is obtained from a simple binary matroid by adding new copies of some vectors or of the zero vector. Two copies of the same nonzero vector in a binary matroid are called parallel, each copy of the zero vector is a loop.
    [Show full text]
  • Introduction to Graph Theory
    Introduction to Graph Theory George Voutsadakis1 1Mathematics and Computer Science Lake Superior State University LSSU Math 351 George Voutsadakis (LSSU) Introduction to Graph Theory August 2018 1 / 41 Outline 1 Matroids Introducing Matroids Examples of Matroids Matroids and Graphs Matroids and Transversals George Voutsadakis (LSSU) Introduction to Graph Theory August 2018 2 / 41 Matroids Introducing Matroids Subsection 1 Introducing Matroids George Voutsadakis (LSSU) Introduction to Graph Theory August 2018 3 / 41 Matroids Introducing Matroids Spanning Trees, Bases and Transversals Recall that a spanning tree in a connected graph G is a connected subgraph of G containing no cycles and including every vertex of G. Note that a spanning tree cannot contain another spanning tree as a proper subgraph. Moreover, if B1 and B2 are spanning trees of G and e is an edge of B1, then there is an edge f in B2, such that (B1 −{e}) ∪{f } (the graph obtained from B1 on replacing e by f ) is also a spanning tree of G. Analogous results hold in the theory of vector spaces and in transversal theory: If V is a vector space and if B1 and B2 are bases of V and e is an element of B1, then we can find an element f of B2 such that (B1 −{e}) ∪{f } is also a basis of V . If E is a set of points and F a collection of subsets of E, T1 and T2 are transversals of F and x an element of T1, there exists an element y of T2, such that (T1 −{x}) ∪{y} is also a transversal of F.
    [Show full text]
  • Chapter 2 Characterizations of Binary Eulerian Matroids*
    CHAPTER 2 CHARACTERIZATIONS OF BINARY EULERIAN MATROIDS* ^ A paper based on the text included in this chapter has been publshed in Indian J. Pure Appl. Math., see Reference No.13 isted at the end. 2.1 INTRODUCTION Eulerian and bipartite matroids have been discussed by Welsh [16]. We recall that a graph is Eulerian if it contains a closed walk that traverses each edge exactly once. It is well known that a graph is Eulerian if and only if its edge set can be partitioned into cycles. Analogously a matroid (S, ^) is Eulerian if there exist disjoint circuits C ,C ,...,C such that 12 ri S = C uC u ...uC 12 n Also a graph is bipartite iff every cycle of it is of even length. Analogously, a matroid (S, 5=") is defined to be bipartite if every circuit has even cardinality. Welsh [16] generalized Euler's well known theorem [see Theorem 1.8] for graphs to binary matroids. In this chapter we generalize Shank's characterization [Theorem 1.11] of Eulerian graphs to binary matroids [Theorem 2.3.1]. The Theorem 2.4.1 extends the characterization of Eulerian graphs [Theorem 1.10] due to Toida and Mckee whereas the Theorem 2.5.1 generalizes results [Theorem 1.13] due to Bondy and Halberstan for graphs. Corresponding results for binary bipartite matroids have been derived. The motivation for these results are from graph theory. They are either analogous to or generalizations of corresponding graph-theoretic results. 13 2.2. EULERIAN AND BIPARTITE MATROIDS We begin with the theorem due to Welsh [16].
    [Show full text]
  • A Characterization of the Graphic Split-Off Matroids
    AUSTRALASIAN JOURNAL OF COMBINATORICS Volume 50 (2011), Pages 25–35 A characterization of the graphic split-off matroids M.M. Shikare∗ S.B. Dhotre K.V. Dalvi Department of Mathematics University of Pune Pune 411 007 India Abstract We consider the problem of determining precisely which graphs G have the property that the split-off operation, by every pair of edges, on the cycle matroid M(G) yields a graphic matroid. This problem is solved by proving that there are exactly four minor-minimal graphs that do not have this property. 1 Introduction The split-off operation for a binary matroid with respect to a pair of elements is defined as follows [9]: Let M be a binary matroid on a set E, and let A be a matrix that represents M over GF (2). Suppose that x, y ∈ E, and the element a ∈ E. Let Axy be the matrix obtained from A by adjoining an extra column, with label a, which is the sum of the columns corresponding to x and y, and then deleting the two columns corresponding to x and y.LetMxy be the vector matroid of the matrix Axy. The transition from M to Mxy is called a split-off operation and the matroid Mxy is referred as the split-off matroid. The split-off operation on a graphic matroid, in general, need not yield a graphic matroid. We characterize precisely those graphic matroids M having the property that for every pair {x, y} of its elements, the matroid Mxy is graphic. This charac- terization is given in terms of the forbidden minors.
    [Show full text]
  • Quaternary Bicycle Matroids and the Penrose Polynomial for Delta-Matroids 3 of the Bicycle Spaces of M Relative to the Subsets Y of the Ground Set of M
    Noname manuscript No. (will be inserted by the editor) Quaternary Bicycle Matroids and the Penrose Polynomial for Delta-Matroids Robert Brijder · Hendrik Jan Hoogeboom Received: date / Accepted: date Abstract In contrast to matroids, vf-safe delta-matroids have three kinds of minors and are closed under the operations of twist and loop complementation. We show that the delta-matroids representable over GF(4) with respect to the nontrivial automor- phism of GF(4) form a subclass of the vf-safe delta-matroids closed under twist and loop complementation. In particular, quaternary matroids are vf-safe. Using this result, we show that the matroid of a bicycle space of a quaternary ma- troid M is obtained from M by using loop complementation. As a consequence, the matroid of a bicycle space of a quaternary matroid M is independent of the chosen representation. This also leads to, e.g., an extension of a known parity-type charac- terization of the bicycle dimension, a generalization of the tripartition of Rosenstiehl and Read [Ann. Disc. Math. (1978)], and a suitable generalization of the dual notions of bipartite and Eulerian binary matroids to a vf-safe delta-matroids. Finally, we generalize a number of results concerning the Penrose polynomial from binary matroids to vf-safe delta-matroids. In this general setting the Penrose polynomial turns out to have a recursive relation much like the recursive relation of the Tutte polynomial. Keywords bicycle matroid · quaternary matroid · Penrose polynomial · delta- matroid · graph tripartition Mathematics Subject Classification (2010) 05C31 · 05B35 · 05C50 · 05C25 R.B. is a postdoctoral fellow of the Research Foundation – Flanders (FWO).
    [Show full text]
  • 36 DM10 Abstracts
    36 DM10 Abstracts IP0 IP3 D´enes K¨onig Prize Lecture: Ramsey Numbers of Extremal Problems for Convex Lattice Polytopes Graphs and Hypergraphs In this survey I will present several extremal problems, and Determining or estimating Ramsey numbers is one of the some solutions, concerning convex lattice polytopes. A typ- central problems in combinatorics. In this talk we discuss ical example is to determine the minimal volume that a recent progress on some longstanding conjectures in this convex lattice polytope can have if it has exactly n ver- area which have played an important role in the develop- tices. Other examples are the minimal surface area, or the ment of Ramsey theory. minimal lattice width in the same class of polytopes. These problems are related to a question of V I Arnold from 1980 Jacob Fox asking for the number of (equivalence classes of) lattice Princeton University polytopes of volume V in d-dimensional space, where two [email protected] convex lattice polytopes are equivalent if one can be carried to the other by a lattice preserving affine transformation. IP1 Imre B´ar´any Binary Matroid Minors R´enyi Institute University College London The graph minors project is a sequence of extraordinary [email protected] theorems proved by Neil Robertson and Paul Seymour in the 1980s. The banner theorem of that project is that every minor-closed family of graphs is characterized by a finite IP4 set of excluded minors. The project also has a number Optimizing in a Strategic World: A Survey of Re- of remarkable algorithmic consequences, for example, the cent Research in Algorithmic Game Theory membership testing problem can be efficiently solved for any minor-closed class of graphs.
    [Show full text]
  • Introduction to Graph Theory
    Introduction to Graph Theory Fourth edition Introduction to Graph Theory Fourth edition Robin J. Wilson Addison Wesley Longman Limited Edinburgh Gate, Harlow, Essex CM20 2JE, England and Associated Companies throughout the world. ©Robin Wilson 1972, 1996 All rights reserved; no part of this publication may be reproduced, stored in any retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise without either the prior written permission of the Publishers or a licence permitting restricted copying in the United Kingdom issued by the Copyright Licensing Agency Ltd., 90 Tottenham Court Road, London W1P9HE. First published by Oliver & Boyd, 1972 Second edition published by Longman Group Ltd, 1979 Third edition, 1985 Fourth edition, 1996 Reprinted 1998 British Library Cataloguing in Publication Data A catalogue record for this book is available from the British Library ISBN 0-582-24993-7 Library of Congress Cataloging-in-Publication Data A catalog record for this book is available from the Library of Congress Set by 8 in 10 on 12pt Times Produced through Longman Malaysia, PP Contents Preface to the fourth edition vii 1 Introduction 1 What is a graph? 1 2 Definitions and examples 2 Definition 8 3 Examples 17 4 Three puzzles 21 3 Paths and cycles 5 Connectivity 26 6 Eulerian graphs 31 7 Hamiltonian graphs 35 8 Some algorithms 38 4 Trees 9 Properties of trees 43 10 Counting trees 47 11 More applications 51 5 Planarity 12 Planar graphs 13 Euler's formula 60 14 Graphs on other surfaces
    [Show full text]
  • How Many Circuits Determine an Oriented Matroid?
    HOW MANY CIRCUITS DETERMINE AN ORIENTED MATROID? KOLJA KNAUER, LUIS PEDRO MONTEJANO, AND JORGE LUIS RAM´IREZ ALFONS´IN This paper is dedicated to the memory of Michel Las Vergnas Abstract. Las Vergnas & Hamidoune studied the number of circuits needed to de- termine an oriented matroid. In this paper we investigate this problem and some new variants, as well as their interpretation in particular classes of matroids. We present general upper and lower bounds in the setting of general connected orientable matroids, leading to the study of subgraphs of the base graph and the intersection graph of circuits. We then consider the problem for uniform matroids which is closely related to the notion of (connected) covering numbers in Design Theory. Finally, we also devote special attention to regular matroids as well as some graphic and cographic matroids leading in particular to the topics of (connected) bond and cycle covers in Graph Theory. 1. Introduction For the general background on matroid and oriented matroid theory we refer the reader to [30] and [4], respectively. An (oriented) matroid is a finite ground set together with a (usually large) set of (oriented) circuits satisfying certain axioms. But, how many of these circuits are actually needed to fully describe a given (oriented) matroid ? In [25, page 721] Lehman shows, that the set Se of circuits of a connected matroid M containing a fixed element e, distinguishes M from all other matroids on the same ground 0 set, that is, if a matroid M on the same ground contains all circuits from Se then M and M0 are the same.
    [Show full text]
  • Circular Flow and Circular Chromatic Number in the Matroid Context
    Circular Flow and Circular Chromatic Number in the Matroid Context by Laura Elena Ch´avez Lomel´ı B.Sc., Universidad Nacional Aut´onoma de M´exico, 1992. M.Sc., University of Oxford, 1994. a Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Mathematics c Laura Elena Ch´avez Lomel´ı 2007 SIMON FRASER UNIVERSITY Spring 2007 All rights reserved. This work may not be reproduced in whole or in part, by photocopy or other means, without the permission of the author. APPROVAL Name: Laura Elena Ch´avez Lomel´ı Degree: Doctor of Philosophy Title of Thesis: Circular Flow and Circular Chromatic Number in the Matroid Context Examining Committee: Dr. Ladislav Stacho Chair Dr. L. A. Goddyn Senior Supervisor Dr. P. Hell Supervisory Committee Dr. P. Lisonek Supervisory Committee Dr. M. Mishna Internal Examiner Dr. D. J. A. Welsh External Examiner Date of Defense: April 2, 2007 ii Abstract This thesis considers circular flow-type and circular chromatic-type parameters (φ and χ, respectively) for matroids. In particular we focus on orientable matroids and √6 1-matroids. These parameters are obtained via two approaches: algebraic and orientation-based. The general questions we discuss are: bounds for flow number; characterizations of Eulerian and bipartite matroids; and possible connections between the two possible extensions of φ: algebraic and orientation. In the case of orientable matroids, we obtain characterizations of bipartite rank-3 matroids and Eulerian uniform, rank-3 matroids; an asymptotic result regarding the flow number of uniform matroids; and an improvement on the known bound for flow number of matroids of arbitrary rank.
    [Show full text]