253 research outputs found
On Binary And Regular Matroids Without Small Minors
The results of this dissertation consist of excluded-minor results for Binary Matroids and excluded-minor results for Regular Matroids. Structural theorems on the relationship between minors and k-sums of matroids are developed here in order to provide some of these characterizations. Chapter 2 of the dissertation contains excluded-minor results for Binary Matroids. The first main result of this dissertation is a characterization of the internally 4-connected binary matroids with no minor that is isomorphic to the cycle matroid of the prism+e graph. This characterization generalizes results of Mayhew and Royle [18] for binary matroids and results of Dirac [8] and Lovász [15] for graphs. The results of this chapter are then extended from the class of internally 4-connected matroids to the class of 3-connected matroids. Chapter 3 of the dissertation contains the second main result, a decomposition theorem for regular matroids without certain minors. This decomposition theorem is used to obtain excluded-minor results for Regular Matroids. Wagner, Lovász, Oxley, Ding, Liu, and others have characterized many classes of graphs that are H-free for graphs H with at most twelve edges (see [7]). We extend several of these excluded-minor characterizations to regular matroids in Chapter 3. We also provide characterizations of regular matroids excluding several graphic matroids such as the octahedron, cube, and the Möbius Ladder on eight vertices. Both theoretical and computer-aided proofs of the results of Chapters 2 and 3 are provided in this dissertation
Interval matroids and graphs
AbstractA base of the cycle space of a binary matroid M on E is said to be convex if its elements can be totally ordered in such a way that for every e ε E the set of elements of the base containing e is an interval. We show that a binary matroid is cographic iff it has a convex base of cycles; equivalently, graphic matroids can be represented as “interval matroids” (matroids associated in a natural way to interval systems). As a consequence, we obtain characterizations of planar graphs and cubic cyclically-4-edge-connected planar graphs in terms of convex bases of cycles
Generalized Laminar Matroids
Nested matroids were introduced by Crapo in 1965 and have appeared frequently
in the literature since then. A flat of a matroid is Hamiltonian if it has
a spanning circuit. A matroid is nested if and only if its Hamiltonian
flats form a chain under inclusion; is laminar if and only if, for every
-element independent set , the Hamiltonian flats of containing
form a chain under inclusion. We generalize these notions to define the classes
of -closure-laminar and -laminar matroids. This paper focuses on
structural properties of these classes noting that, while the second class is
always minor-closed, the first is if and only if . The main results
are excluded-minor characterizations for the classes of 2-laminar and
2-closure-laminar matroids.Comment: 12 page
Laminar Matroids
A laminar family is a collection of subsets of a set such
that, for any two intersecting sets, one is contained in the other. For a
capacity function on , let be \{I:|I\cap A|
\leq c(A)\text{ for all A\in\mathscr{A}}\}. Then is the
collection of independent sets of a (laminar) matroid on . We present a
method of compacting laminar presentations, characterize the class of laminar
matroids by their excluded minors, present a way to construct all laminar
matroids using basic operations, and compare the class of laminar matroids to
other well-known classes of matroids.Comment: 17 page
Graphical representations of graphic frame matroids
A frame matroid M is graphic if there is a graph G with cycle matroid
isomorphic to M. In general, if there is one such graph, there will be many.
Zaslavsky has shown that frame matroids are precisely those having a
representation as a biased graph; this class includes graphic matroids,
bicircular matroids, and Dowling geometries. Whitney characterized which graphs
have isomorphic cycle matroids, and Matthews characterised which graphs have
isomorphic graphic bicircular matroids. In this paper, we give a
characterization of which biased graphs give rise to isomorphic graphic frame
matroids
- …