425 research outputs found
Hamiltonicity and -hypergraphs
We define and study a special type of hypergraph. A -hypergraph ), where is a partition of , is an
-uniform hypergraph having vertices partitioned into classes of
vertices each. If the classes are denoted by , ,...,, then a
subset of of size is an edge if the partition of formed by
the non-zero cardinalities , ,
is . The non-empty intersections are called the parts
of , and denotes the number of parts. We consider various types
of cycles in hypergraphs such as Berge cycles and sharp cycles in which only
consecutive edges have a nonempty intersection. We show that most
-hypergraphs contain a Hamiltonian Berge cycle and that, for and , a -hypergraph always contains a sharp
Hamiltonian cycle. We also extend this result to -intersecting cycles
Loose Hamiltonian cycles forced by large -degree - sharp version
We prove for all and the sharp minimum
-degree bound for a -uniform hypergraph on vertices
to contain a Hamiltonian -cycle if divides and is
sufficiently large. This extends a result of Han and Zhao for -uniform
hypegraphs.Comment: 14 pages, second version addresses changes arising from the referee
report
Constructing Union-Free pairs of K-Element subsets
It is proved that one can choose [1/2(n/k)] disjoint pairs of k-element subsets of an n-element set in such a way that the unions of the pairs are all different, supposing that n > n(k)
Tight Euler tours in uniform hypergraphs - computational aspects
By a tight tour in a -uniform hypergraph we mean any sequence of its
vertices such that for all the set
is an edge of (where operations on
indices are computed modulo ) and the sets for are
pairwise different. A tight tour in is a tight Euler tour if it contains
all edges of . We prove that the problem of deciding if a given -uniform
hypergraph has a tight Euler tour is NP-complete, and that it cannot be solved
in time (where is the number of edges in the input hypergraph),
unless the ETH fails. We also present an exact exponential algorithm for the
problem, whose time complexity matches this lower bound, and the space
complexity is polynomial. In fact, this algorithm solves a more general problem
of computing the number of tight Euler tours in a given uniform hypergraph
Hamilton cycles in graphs and hypergraphs: an extremal perspective
As one of the most fundamental and well-known NP-complete problems, the
Hamilton cycle problem has been the subject of intensive research. Recent
developments in the area have highlighted the crucial role played by the
notions of expansion and quasi-randomness. These concepts and other recent
techniques have led to the solution of several long-standing problems in the
area. New aspects have also emerged, such as resilience, robustness and the
study of Hamilton cycles in hypergraphs. We survey these developments and
highlight open problems, with an emphasis on extremal and probabilistic
approaches.Comment: to appear in the Proceedings of the ICM 2014; due to given page
limits, this final version is slightly shorter than the previous arxiv
versio
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