288 research outputs found
Planar 3-SAT with a Clause/Variable Cycle
In the Planar 3-SAT problem, we are given a 3-SAT formula together with its
incidence graph, which is planar, and are asked whether this formula is
satisfiable. Since Lichtenstein's proof that this problem is NP-complete, it
has been used as a starting point for a large number of reductions. In the
course of this research, different restrictions on the incidence graph of the
formula have been devised, for which the problem also remains hard.
In this paper, we investigate the restriction in which we require that the
incidence graph can be augmented by the edges of a Hamiltonian cycle that first
passes through all variables and then through all clauses, in a way that the
resulting graph is still planar. We show that the problem of deciding
satisfiability of a 3-SAT formula remains NP-complete even if the incidence
graph is restricted in that way and the Hamiltonian cycle is given. This
complements previous results demanding cycles only through either the variables
or clauses.
The problem remains hard for monotone formulas, as well as for instances with
exactly three distinct variables per clause. In the course of this
investigation, we show that monotone instances of Planar 3-SAT with exactly
three distinct variables per clause are always satisfiable, thus settling the
question by Darmann, D\"ocker, and Dorn on the complexity of this problem
variant in a surprising way.Comment: Implementing style of DMTCS journa
Induced Ramsey-type results and binary predicates for point sets
Let and be positive integers and let be a finite point set in
general position in the plane. We say that is -Ramsey if there is a
finite point set such that for every -coloring of
there is a subset of such that and have the same order type
and is monochromatic in . Ne\v{s}et\v{r}il and Valtr proved
that for every , all point sets are -Ramsey. They also
proved that for every and , there are point sets that are
not -Ramsey.
As our main result, we introduce a new family of -Ramsey point sets,
extending a result of Ne\v{s}et\v{r}il and Valtr. We then use this new result
to show that for every there is a point set such that no function
that maps ordered pairs of distinct points from to a set of size
can satisfy the following "local consistency" property: if attains
the same values on two ordered triples of points from , then these triples
have the same orientation. Intuitively, this implies that there cannot be such
a function that is defined locally and determines the orientation of point
triples.Comment: 22 pages, 3 figures, final version, minor correction
Order on Order Types
Given P and P\u27, equally sized planar point sets in general position, we call a bijection from P to P\u27 crossing-preserving if crossings of connecting segments in P are preserved in P\u27 (extra crossings may occur in P\u27). If such a mapping exists, we say that P\u27 crossing-dominates P, and if such a mapping exists in both directions, P and P\u27 are called crossing-equivalent. The relation is transitive, and we have a partial order on the obtained equivalence classes (called crossing types or x-types). Point sets of equal order type are clearly crossing-equivalent, but not vice versa. Thus, x-types are a coarser classification than order types. (We will see, though, that a collapse of different order types to one x-type occurs for sets with triangular convex hull only.)
We argue that either the maximal or the minimal x-types are sufficient for answering many combinatorial (existential or extremal) questions on planar point sets. Motivated by this we consider basic properties of the relation. We characterize order types crossing-dominated by points in convex position. Further, we give a full characterization of minimal and maximal abstract order types. Based on that, we provide a polynomial-time algorithm to check whether a point set crossing-dominates another. Moreover, we generate all maximal and minimal x-types for small numbers of points
Linear transformation distance for bichromatic matchings
Let be a set of points in general position, where is a
set of blue points and a set of red points. A \emph{-matching}
is a plane geometric perfect matching on such that each edge has one red
endpoint and one blue endpoint. Two -matchings are compatible if their
union is also plane.
The \emph{transformation graph of -matchings} contains one node for each
-matching and an edge joining two such nodes if and only if the
corresponding two -matchings are compatible. In SoCG 2013 it has been shown
by Aloupis, Barba, Langerman, and Souvaine that this transformation graph is
always connected, but its diameter remained an open question. In this paper we
provide an alternative proof for the connectivity of the transformation graph
and prove an upper bound of for its diameter, which is asymptotically
tight
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