96 research outputs found
A proof of the rooted tree alternative conjecture
Bonato and Tardif conjectured that the number of isomorphism classes of trees
mutually embeddable with a given tree T is either 1 or infinite. We prove the
analogue of their conjecture for rooted trees. We also discuss the original
conjecture for locally finite trees and state some new conjectures
On tree-decompositions of one-ended graphs
A graph is one-ended if it contains a ray (a one way infinite path) and
whenever we remove a finite number of vertices from the graph then what remains
has only one component which contains rays. A vertex {\em dominates} a ray
in the end if there are infinitely many paths connecting to the ray such
that any two of these paths have only the vertex in common. We prove that
if a one-ended graph contains no ray which is dominated by a vertex and no
infinite family of pairwise disjoint rays, then it has a tree-decomposition
such that the decomposition tree is one-ended and the tree-decomposition is
invariant under the group of automorphisms.
This can be applied to prove a conjecture of Halin from 2000 that the
automorphism group of such a graph cannot be countably infinite and solves a
recent problem of Boutin and Imrich. Furthermore, it implies that every
transitive one-ended graph contains an infinite family of pairwise disjoint
rays
Extremal Infinite Graph Theory
We survey various aspects of infinite extremal graph theory and prove several
new results. The lead role play the parameters connectivity and degree. This
includes the end degree. Many open problems are suggested.Comment: 41 pages, 16 figure
Invariant subsets of scattered trees. An application to the tree alternative property of Bonato and Tardif
A tree is scattered if no subdivision of the complete binary tree is a
subtree. Building on results of Halin, Polat and Sabidussi, we identify four
types of subtrees of a scattered tree and a function of the tree into the
integers at least one of which is preserved by every embedding.
With this result and a result of Tyomkyn, we prove that the tree alternative
property conjecture of Bonato and Tardif holds for scattered trees and a
conjecture of Tyomkin holds for locally finite scattered trees
Even an infinite bureaucracy eventually makes a decision
We show that the fact that a political decision filtered through a finite
tree of committees gives a determined answer generalises in some sense to
infinite trees. This implies a new special case of the Matroid Intersection
Conjecture
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