1,206 research outputs found
Fork-decompositions of matroids
For the abstract of this paper, please see the PDF file
Constructing internally 4-connected binary matroids
This is the post-print version of the Article - Copyright @ 2013 ElsevierIn an earlier paper, we proved that an internally 4-connected binary matroid with at least seven elements contains an internally 4-connected proper minor that is at most six elements smaller. We refine this result, by giving detailed descriptions of the operations required to produce the internally 4-connected minor. Each of these operations is top-down, in that it produces a smaller minor from the original. We also describe each as a bottom-up operation, constructing a larger matroid from the original, and we give necessary and su fficient conditions for each of these bottom-up moves to produce an internally 4-connected binary matroid. From this, we derive a constructive method for generating all internally 4-connected binary matroids.This study is supported by NSF IRFP Grant 0967050, the Marsden Fund, and the National Security Agency
Matroids with at least two regular elements
For a matroid , an element such that both and
are regular is called a regular element of . We determine completely the
structure of non-regular matroids with at least two regular elements. Besides
four small size matroids, all 3-connected matroids in the class can be pieced
together from or and a regular matroid using 3-sums. This result
takes a step toward solving a problem posed by Paul Seymour: Find all
3-connected non-regular matroids with at least one regular element [5, 14.8.8]
On Density-Critical Matroids
For a matroid having rank-one flats, the density is
unless , in which case . A matroid is
density-critical if all of its proper minors of non-zero rank have lower
density. By a 1965 theorem of Edmonds, a matroid that is minor-minimal among
simple matroids that cannot be covered by independent sets is
density-critical. It is straightforward to show that is the only
minor-minimal loopless matroid with no covering by independent sets. We
prove that there are exactly ten minor-minimal simple obstructions to a matroid
being able to be covered by two independent sets. These ten matroids are
precisely the density-critical matroids such that but for all proper minors of . All density-critical matroids of density
less than are series-parallel networks. For , although finding all
density-critical matroids of density at most does not seem straightforward,
we do solve this problem for .Comment: 16 page
Triangle-roundedness in matroids
A matroid is said to be triangle-rounded in a class of matroids
if each -connected matroid with a triangle
and an -minor has an -minor with as triangle. Reid gave a result
useful to identify such matroids as stated next: suppose that is a binary
-connected matroid with a -connected minor , is a triangle of
and ; then has a -connected minor with an
-minor such that is a triangle of and . We
strengthen this result by dropping the condition that such element exists
and proving that there is a -connected minor of with an -minor
such that is a triangle of and . This
result is extended to the non-binary case and, as an application, we prove that
is triangle-rounded in the class of the regular matroids
Internally 4-connected binary matroids with cyclically sequential orderings
We characterize all internally 4-connected binary matroids M with the property that the ground set of M can be ordered (e0,…,en−1) in such a way that {ei,…,ei+t} is 4-separating for all 0≤i,t≤n−1 (all subscripts are read modulo n). We prove that in this case either n≤7 or, up to duality, M is isomorphic to the polygon matroid of a cubic or quartic planar ladder, the polygon matroid of a cubic or quartic Möbius ladder, a particular single-element extension of a wheel, or a particular single-element extension of the bond matroid of a cubic ladder
A chain theorem for internally 4-connected binary matroids
This is the post-print version of the Article - Copyright @ 2011 ElsevierLet M be a matroid. When M is 3-connected, Tutte’s Wheels-and-Whirls Theorem proves that M has a 3-connected proper minor N with |E(M) − E(N)| = 1 unless M is a wheel or a whirl. This paper establishes a corresponding result for internally 4-connected binary matroids. In particular, we prove that if M is such a matroid, then M has an internally 4-connected proper minor N with |E(M) − E(N)| at most 3 unless M or its dual is the cycle matroid of a planar or Möbius quartic ladder, or a 16-element variant of such a planar ladder.This study was partially supported by the National Security Agency
Dessins, their delta-matroids and partial duals
Given a map on a connected and closed orientable surface, the
delta-matroid of is a combinatorial object associated to which captures some topological information of the embedding. We explore how
delta-matroids associated to dessins d'enfants behave under the action of the
absolute Galois group. Twists of delta-matroids are considered as well; they
correspond to the recently introduced operation of partial duality of maps.
Furthermore, we prove that every map has a partial dual defined over its field
of moduli. A relationship between dessins, partial duals and tropical curves
arising from the cartography groups of dessins is observed as well.Comment: 34 pages, 20 figures. Accepted for publication in the SIGMAP14
Conference Proceeding
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