111 research outputs found
Computing metric hulls in graphs
We prove that, given a closure function the smallest preimage of a closed set
can be calculated in polynomial time in the number of closed sets. This
confirms a conjecture of Albenque and Knauer and implies that there is a
polynomial time algorithm to compute the convex hull-number of a graph, when
all its convex subgraphs are given as input. We then show that computing if the
smallest preimage of a closed set is logarithmic in the size of the ground set
is LOGSNP-complete if only the ground set is given. A special instance of this
problem is computing the dimension of a poset given its linear extension graph,
that was conjectured to be in P.
The intent to show that the latter problem is LOGSNP-complete leads to
several interesting questions and to the definition of the isometric hull,
i.e., a smallest isometric subgraph containing a given set of vertices .
While for an isometric hull is just a shortest path, we show that
computing the isometric hull of a set of vertices is NP-complete even if
. Finally, we consider the problem of computing the isometric
hull-number of a graph and show that computing it is complete.Comment: 13 pages, 3 figure
Cuts in matchings of 3-connected cubic graphs
We discuss conjectures on Hamiltonicity in cubic graphs (Tait, Barnette,
Tutte), on the dichromatic number of planar oriented graphs (Neumann-Lara), and
on even graphs in digraphs whose contraction is strongly connected
(Hochst\"attler). We show that all of them fit into the same framework related
to cuts in matchings. This allows us to find a counterexample to the conjecture
of Hochst\"attler and show that the conjecture of Neumann-Lara holds for all
planar graphs on at most 26 vertices. Finally, we state a new conjecture on
bipartite cubic oriented graphs, that naturally arises in this setting.Comment: 12 pages, 5 figures, 1 table. Improved expositio
Chomp on numerical semigroups
We consider the two-player game chomp on posets associated to numerical
semigroups and show that the analysis of strategies for chomp is strongly
related to classical properties of semigroups. We characterize, which player
has a winning-strategy for symmetric semigroups, semigroups of maximal
embedding dimension and several families of numerical semigroups generated by
arithmetic sequences. Furthermore, we show that which player wins on a given
numerical semigroup is a decidable question. Finally, we extend several of our
results to the more general setting of subsemigroups of ,
where is a finite abelian group.Comment: 22 pages, 14 figures, 1 table (improved exposition
Hypercellular graphs: partial cubes without as partial cube minor
We investigate the structure of isometric subgraphs of hypercubes (i.e.,
partial cubes) which do not contain finite convex subgraphs contractible to the
3-cube minus one vertex (here contraction means contracting the edges
corresponding to the same coordinate of the hypercube). Extending similar
results for median and cellular graphs, we show that the convex hull of an
isometric cycle of such a graph is gated and isomorphic to the Cartesian
product of edges and even cycles. Furthermore, we show that our graphs are
exactly the class of partial cubes in which any finite convex subgraph can be
obtained from the Cartesian products of edges and even cycles via successive
gated amalgams. This decomposition result enables us to establish a variety of
results. In particular, it yields that our class of graphs generalizes median
and cellular graphs, which motivates naming our graphs hypercellular.
Furthermore, we show that hypercellular graphs are tope graphs of zonotopal
complexes of oriented matroids. Finally, we characterize hypercellular graphs
as being median-cell -- a property naturally generalizing the notion of median
graphs.Comment: 35 pages, 6 figures, added example answering Question 1 from earlier
draft (Figure 6.
Classification of coupled dynamical systems with multiple delays: Finding the minimal number of delays
In this article we study networks of coupled dynamical systems with
time-delayed connections. If two such networks hold different delays on the
connections it is in general possible that they exhibit different dynamical
behavior as well. We prove that for particular sets of delays this is not the
case. To this aim we introduce a componentwise timeshift transformation (CTT)
which allows to classify systems which possess equivalent dynamics, though
possibly different sets of connection delays. In particular, we show for a
large class of semiflows (including the case of delay differential equations)
that the stability of attractors is invariant under this transformation.
Moreover we show that each equivalence class which is mediated by the CTT
possesses a representative system in which the number of different delays is
not larger than the cycle space dimension of the underlying graph. We conclude
that the 'true' dimension of the corresponding parameter space of delays is in
general smaller than it appears at first glance
COMs: Complexes of Oriented Matroids
In his seminal 1983 paper, Jim Lawrence introduced lopsided sets and featured
them as asymmetric counterparts of oriented matroids, both sharing the key
property of strong elimination. Moreover, symmetry of faces holds in both
structures as well as in the so-called affine oriented matroids. These two
fundamental properties (formulated for covectors) together lead to the natural
notion of "conditional oriented matroid" (abbreviated COM). These novel
structures can be characterized in terms of three cocircuits axioms,
generalizing the familiar characterization for oriented matroids. We describe a
binary composition scheme by which every COM can successively be erected as a
certain complex of oriented matroids, in essentially the same way as a lopsided
set can be glued together from its maximal hypercube faces. A realizable COM is
represented by a hyperplane arrangement restricted to an open convex set. Among
these are the examples formed by linear extensions of ordered sets,
generalizing the oriented matroids corresponding to the permutohedra. Relaxing
realizability to local realizability, we capture a wider class of combinatorial
objects: we show that non-positively curved Coxeter zonotopal complexes give
rise to locally realizable COMs.Comment: 40 pages, 6 figures, (improved exposition
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