140 research outputs found
Disjoint compatibility graph of non-crossing matchings of points in convex position
Let be a set of labeled points in convex position in the plane.
We consider geometric non-intersecting straight-line perfect matchings of
. Two such matchings, and , are disjoint compatible if they do
not have common edges, and no edge of crosses an edge of . Denote by
the graph whose vertices correspond to such matchings, and two
vertices are adjacent if and only if the corresponding matchings are disjoint
compatible. We show that for each , the connected components of
form exactly three isomorphism classes -- namely, there is a
certain number of isomorphic small components, a certain number of isomorphic
medium components, and one big component. The number and the structure of small
and medium components is determined precisely.Comment: 46 pages, 30 figure
Quasi-Parallel Segments and Characterization of Unique Bichromatic Matchings
Given n red and n blue points in general position in the plane, it is
well-known that there is a perfect matching formed by non-crossing line
segments. We characterize the bichromatic point sets which admit exactly one
non-crossing matching. We give several geometric descriptions of such sets, and
find an O(nlogn) algorithm that checks whether a given bichromatic set has this
property.Comment: 31 pages, 24 figure
Compatible matchings in geometric graphs
Two non-crossing geometric graphs on the same set of points are compatible if their union
is also non-crossing. In this paper, we prove that every graph G that has an outerplanar embedding
admits a non-crossing perfect matching compatible with G. Moreover, for non-crossing geometric trees
and simple polygons, we study bounds on the minimum number of edges that a compatible non-crossing
perfect matching must share with the tree or the polygon. We also give bounds on the maximal size of
a compatible matching (not necessarily perfect) that is disjoint from the tree or the polygon.Postprint (published version
On Compatible Matchings
A matching is compatible to two or more labeled point sets of size with
labels if its straight-line drawing on each of these point sets
is crossing-free. We study the maximum number of edges in a matching compatible
to two or more labeled point sets in general position in the plane. We show
that for any two labeled convex sets of points there exists a compatible
matching with edges. More generally, for any
labeled point sets we construct compatible matchings of size
. As a corresponding upper bound, we use probabilistic
arguments to show that for any given sets of points there exists a
labeling of each set such that the largest compatible matching has
edges. Finally, we show that
copies of any set of points are necessary and sufficient for the existence
of a labeling such that any compatible matching consists only of a single edge
Flip Distance to some Plane Configurations
We study an old geometric optimization problem in the plane. Given a perfect matching M on a set of n points in the plane, we can transform it to a non-crossing perfect matching by a finite sequence of flip operations. The flip operation removes two crossing edges from M and adds two non-crossing edges. Let f(M) and F(M) denote the minimum and maximum lengths of a flip sequence on M, respectively. It has been proved by Bonnet and Miltzow (2016) that f(M)=O(n^2) and by van Leeuwen and Schoone (1980) that F(M)=O(n^3). We prove that f(M)=O(n Delta) where Delta is the spread of the point set, which is defined as the ratio between the longest and the shortest pairwise distances. This improves the previous bound for point sets with sublinear spread. For a matching M on n points in convex position we prove that f(M)=n/2-1 and F(M)={{n/2} choose 2}; these bounds are tight.
Any bound on F(*) carries over to the bichromatic setting, while this is not necessarily true for f(*). Let M\u27 be a bichromatic matching. The best known upper bound for f(M\u27) is the same as for F(M\u27), which is essentially O(n^3). We prove that f(M\u27)<=slant n-2 for points in convex position, and f(M\u27)= O(n^2) for semi-collinear points.
The flip operation can also be defined on spanning trees. For a spanning tree T on a convex point set we show that f(T)=O(n log n)
Flip Distance to a Non-crossing Perfect Matching
A perfect straight-line matching on a finite set of points in the
plane is a set of segments such that each point in is an endpoint of
exactly one segment. is non-crossing if no two segments in cross each
other. Given a perfect straight-line matching with at least one crossing,
we can remove this crossing by a flip operation. The flip operation removes two
crossing segments on a point set and adds two non-crossing segments to
attain a new perfect matching . It is well known that after a finite number
of flips, a non-crossing matching is attained and no further flip is possible.
However, prior to this work, no non-trivial upper bound on the number of flips
was known. If (resp.~) is the maximum length of the longest
(resp.~shortest) sequence of flips starting from any matching of size , we
show that and (resp.~ and
)
IST Austria Thesis
This thesis considers two examples of reconfiguration problems: flipping edges in edge-labelled triangulations of planar point sets and swapping labelled tokens placed on vertices of a graph. In both cases the studied structures – all the triangulations of a given point set or all token placements on a given graph – can be thought of as vertices of the so-called reconfiguration graph, in which two vertices are adjacent if the corresponding structures differ by a single elementary operation – by a flip of a diagonal in a triangulation or by a swap of tokens on adjacent vertices, respectively. We study the reconfiguration of one instance of a structure into another via (shortest) paths in the reconfiguration graph.
For triangulations of point sets in which each edge has a unique label and a flip transfers the label from the removed edge to the new edge, we prove a polynomial-time testable condition, called the Orbit Theorem, that characterizes when two triangulations of the same point set lie in the same connected component of the reconfiguration graph. The condition was first conjectured by Bose, Lubiw, Pathak and Verdonschot. We additionally provide a polynomial time algorithm that computes a reconfiguring flip sequence, if it exists. Our proof of the Orbit Theorem uses topological properties of a certain high-dimensional cell complex that has the usual reconfiguration graph as its 1-skeleton.
In the context of token swapping on a tree graph, we make partial progress on the problem of finding shortest reconfiguration sequences. We disprove the so-called Happy Leaf Conjecture and demonstrate the importance of swapping tokens that are already placed at the correct vertices. We also prove that a generalization of the problem to weighted coloured token swapping is NP-hard on trees but solvable in polynomial time on paths and stars
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