60 research outputs found
Resolution of the Oberwolfach problem
The Oberwolfach problem, posed by Ringel in 1967, asks for a decomposition of
into edge-disjoint copies of a given -factor. We show that this
can be achieved for all large . We actually prove a significantly more
general result, which allows for decompositions into more general types of
factors. In particular, this also resolves the Hamilton-Waterloo problem for
large .Comment: 28 page
Minimalist designs
The iterative absorption method has recently led to major progress in the
area of (hyper-)graph decompositions. Amongst other results, a new proof of the
Existence conjecture for combinatorial designs, and some generalizations, was
obtained. Here, we illustrate the method by investigating triangle
decompositions: we give a simple proof that a triangle-divisible graph of large
minimum degree has a triangle decomposition and prove a similar result for
quasi-random host graphs.Comment: updated references, to appear in Random Structures & Algorithm
Hamilton decompositions of regular expanders: a proof of Kelly's conjecture for large tournaments
A long-standing conjecture of Kelly states that every regular tournament on n
vertices can be decomposed into (n-1)/2 edge-disjoint Hamilton cycles. We prove
this conjecture for large n. In fact, we prove a far more general result, based
on our recent concept of robust expansion and a new method for decomposing
graphs. We show that every sufficiently large regular digraph G on n vertices
whose degree is linear in n and which is a robust outexpander has a
decomposition into edge-disjoint Hamilton cycles. This enables us to obtain
numerous further results, e.g. as a special case we confirm a conjecture of
Erdos on packing Hamilton cycles in random tournaments. As corollaries to the
main result, we also obtain several results on packing Hamilton cycles in
undirected graphs, giving e.g. the best known result on a conjecture of
Nash-Williams. We also apply our result to solve a problem on the domination
ratio of the Asymmetric Travelling Salesman problem, which was raised e.g. by
Glover and Punnen as well as Alon, Gutin and Krivelevich.Comment: new version includes a standalone version of the `robust
decomposition lemma' for application in subsequent paper
Combinatorics
This is the report on the Oberwolfach workshop on Combinatorics, held 1–7 January 2006. Combinatorics is a branch of mathematics studying families of mainly, but not exclusively, finite or countable structures – discrete objects. The discrete objects considered in the workshop were graphs, set systems, discrete geometries, and matrices. The programme consisted of 15 invited lectures, 18 contributed talks, and a problem session focusing on recent developments in graph theory, coding theory, discrete geometry, extremal combinatorics, Ramsey theory, theoretical computer science, and probabilistic combinatorics
Thresholds for Latin squares and Steiner triple systems: Bounds within a logarithmic factor
We prove that for and an absolute constant , if and is a random subset of where
each is included in independently with probability for
each , then asymptotically almost surely there is an order-
Latin square in which the entry in the th row and th column lies in
. The problem of determining the threshold probability for the
existence of an order- Latin square was raised independently by Johansson,
by Luria and Simkin, and by Casselgren and H{\"a}ggkvist; our result provides
an upper bound which is tight up to a factor of and strengthens the
bound recently obtained by Sah, Sawhney, and Simkin. We also prove analogous
results for Steiner triple systems and -factorizations of complete graphs,
and moreover, we show that each of these thresholds is at most the threshold
for the existence of a -factorization of a nearly complete regular bipartite
graph.Comment: 32 pages, 1 figure. Final version, to appear in Transactions of the
AM
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