3 research outputs found
Infinite Gammoids: Minors and Duality
This sequel to our paper (Infinite gammoids, 2014) considers minors and duals
of infinite gammoids. We prove that a class of gammoids definable by digraphs
not containing a certain type of substructure, called an outgoing comb, is
minor-closed. Also, we prove that finite-rank minors of gammoids are gammoids.
Furthermore, the topological gammoids introduced by Carmesin (Topological
infinite gammoids, and a new Menger-type theorem for infinite graphs, 2014) are
proved to coincide, as matroids, with the finitary gammoids. A corollary is
that topological gammoids are minor-closed. It is a well-known fact that the
dual of any finite strict gammoid is a transversal matroid. The class of
alternating-comb-free strict gammoids, introduced in the prequel, contains
examples which are not dual to any transversal matroid. However, we describe
the duals of matroids in this class as a natural extension of transversal
matroids. While finite gammoids are closed under duality, we construct a strict
gammoid that is not dual to any gammoid
Nearly Finitary Matroids
In this thesis, we study nearly finitary matroids by introducing new
definitions and prove various properties of nearly finitary matroids. In 2010,
an axiom system for infinite matroids was proposed by Bruhn et al. We use this
axiom system for this thesis. In Chapter 2, we summarize our main results after
reviewing historical background and motivation. In Chapter 3, we define a
notion of spectrum for matroids. Moreover, we show that the spectrum of a
nearly finitary matroid can be larger than any fixed finite size. We also give
an example of a matroid with infinitely large spectrum that is not nearly
finitary. Assuming the existence of a single matroid that is nearly finitary
but not -nearly finitary, we construct classes of matroids that are nearly
finitary but not -nearly finitary. We also show that finite rank matroids
are unionable. In Chapter 4, we will introduce a notion of near finitarization.
We also give an example of a nearly finitary independence system that is not
-nearly finitary. This independence system is not a matroid. In Chapter 5,
we will talk about Psi-matroids and introduce a possible generalization.
Moreover, we study these new matroids to search for an example of a nearly
finitary matroid that is not -nearly finitary. We have not yet found such an
example. In Chapter 6, we will discuss thin sums matroids and consider our
problem restricted to this class of matroids. Our results are motivated by the
open problem concerning whether every nearly finitary matroid is -nearly
finitary for some .Comment: PhD Thesis, UC Davis (2018