Triangle-roundedness in matroids

A matroid $N$ is said to be triangle-rounded in a class of matroids $\mathcal{M}$ if each $3$-connected matroid $M\in \mathcal{M}$ with a triangle $T$ and an $N$-minor has an $N$-minor with $T$ as triangle. Reid gave a result useful to identify such matroids as stated next: suppose that $M$ is a binary $3$-connected matroid with a $3$-connected minor $N$, $T$ is a triangle of $M$ and $e\in T\cap E(N)$; then $M$ has a $3$-connected minor $M'$ with an $N$-minor such that $T$ is a triangle of $M'$ and $|E(M')|\le |E(N)|+2$. We strengthen this result by dropping the condition that such element $e$ exists and proving that there is a $3$-connected minor $M'$ of $M$ with an $N$-minor $N'$ such that $T$ is a triangle of $M'$ and $E(M')-E(N')\subseteq T$. This result is extended to the non-binary case and, as an application, we prove that $M(K_5)$ is triangle-rounded in the class of the regular matroids

A notion of minor-based matroid connectivity

For a matroid $N$, a matroid $M$ is $N$-connected if every two elements of $M$ are in an $N$-minor together. Thus a matroid is connected if and only if it is $U_{1,2}$-connected. This paper proves that $U_{1,2}$ is the only connected matroid $N$ such that if $M$ is $N$-connected with $|E(M)| > |E(N)|$, then $M \backslash e$ or $M / e$ is $N$-connected for all elements $e$. Moreover, we show that $U_{1,2}$ and $M(\mathcal{W}_2)$ are the only connected matroids $N$ such that, whenever a matroid has an $N$-minor using $\{e,f\}$ and an $N$-minor using $\{f,g\}$, it also has an $N$-minor using $\{e,g\}$. Finally, we show that $M$ is $U_{0,1} \oplus U_{1,1}$-connected if and only if every clonal class of $M$ is trivial.Comment: 13 page

Triangles in 3-connected matroids

AbstractA collection F of 3-connected matroids is triangle-rounded if, whenever M is a 3-connected matroid having a minor in F, and T is a 3-element circuit of M, then M has a minor which uses T and is isomorphic to a member of F. An efficient theorem for testing a collection of matroids for this property is presented. This test is used to obtain several results including the following extension of a result of Asano, Nishizeki, and Seymour. Let T be a 3-element circuit of a 3-connected binary nonregular matroid M with at least eight elements. Then M has a minor using T that is isomorphic to S8 or the generalized parallel connection across T of F7 and M(K4)

The Smallest Rounded Sets of Binary Matroids

It was proved by Oxley that U2,4 is the only non-trivial 3-connected matroid N such that, whenever a 3-connected matroid M has an N-minor and x and y are elements of M, there is an N-minor of M using {x, y} . This paper establishes the corresponding result for binary matroids by proving that if M and N above must both be binary, then there are exactly two possibilities for N: the rank-three and rank-four wheels. © 1990, Academic Press Limited. All rights reserved

On Minors Avoiding Elements in Matroids

Let ℱ be a collection of 3-connected matroids, none a proper minor of another, such that if M is a 3-connected matroid having a proper ℱ-minor and e is an element of M, then M has an ℱ-minor avoiding e. This paper proves that there are precisely two collections ℱ with this property: {U2,4} and {U2,4, M(K4)}. Several extensions of this result and some similar results for 2-connected matroids are also established. © 1991, Academic Press Limited. All rights reserved

On inequivalent representations of matroids over finite fields

Kahn conjectured in 1988 that, for each prime power q, there is an integer n(q) such that no 3-connected GF(q)-representable matroid has more than n(q) inequivalent GF(q)-representations. At the time, this conjecture was known to be true for q = 2 and q = 3, and Kahn had just proved it for q = 4. In this paper, we prove the conjecture for q = 5, showing that 6 is a sharp value for n(5). Moreover, we also show that the conjecture is false for all larger values of q. © 1996 Academic Press, Inc

On Roundedness in Matroid Theory.

This thesis studies the relationship between subsets and specified minors in a 3-connected matroid. For positive integers k and m, a set S of k-connected matroids is (k,m)-rounded if it satisfies the following condition. Whenever M is a k-connected matroid having an S-minor and X is a subset of E(M) with at most m elements, then M has an S-minor using X. Oxley characterized the (3,2)-rounded sets that contain a single matroid. In Chapter 2, we obtain an analog of this result for binary matroids. In Chapter 3, we use this result to characterize the pairs of matroids which form (3,2)-rounded sets. The methods of Chapter 3 are generalized to 4-connected matroids in Chapter 4 to determine the (4,2)-rounded sets that contain a single matroid. This extends results of Coullard and Kahn. For a 3-connected minor N of a 3-connected matroid M, the following question arises from roundedness theory. Let X be a subset of E(M). How small a 3-connected minor of M can we find which both uses X and has an N-minor? Seymour answered this question for $\vert$X$\vert$ = 1 and 2. We answer this question for $\vert$X$\vert$ $\geq$ 3 in Chapter 5. Finally, in Chapter 6, results from roundedness theory are applied to the study of 3-element circuits in 3-connected matroids. An extension of a result of Asano, Nishizeki, and Seymour is obtained for binary matroids which are non-regular

Structure and Minors in Graphs and Matroids.

This dissertation establishes a number of theorems related to the structure of graphs and, more generally, matroids. In Chapter 2, we prove that a 3-connected graph G that has a triangle in which every single-edge contraction is 3-connected has a minor that uses the triangle and is isomorphic to K5 or the octahedron. We subsequently extend this result to the more general context of matroids. In Chapter 3, we specifically consider the triangle-rounded property that emerges in the results of Chapter 2. In particular, Asano, Nishizeki, and Seymour showed that whenever a 3-connected matroid M has a four-point-line-minor, and T is a triangle of M, there is a four-point-line-minor of M using T. We will prove that the four-point line is the only such matroid on at least four elements. In Chapter 4, we extend a result of Dirac which states that any set of n vertices of an n-connected graph lies in a cycle. We prove that if V\u27 is a set of at most 2n vertices in an n-connected graph G, then G has, as a minor, a cycle using all of the vertices of V\u27. In Chapter 5, we prove that, for any vertex v of an n-connected simple graph G, there is a n-spoked-wheel-minor of G using v and any n edges incident with v. We strengthen this result in the context of 4-connected graphs by proving that, for any vertex v of a 4-connected simple graph G, there is a K 5- or octahedron-minor of G using v and any four edges incident with v. Motivated by the results of Chapters 4 and 5, in Chapter 6, we introduce the concept of vertex-roundedness. Specifically, we provide a finite list of conditions under which one can determine which collections of graphs have the property that whenever a sufficiently highly connected graph has a minor in the collection, it has such a minor using any set of vertices of some fixed size