48 research outputs found
Geometric Embeddability of Complexes Is ??-Complete
We show that the decision problem of determining whether a given (abstract simplicial) k-complex has a geometric embedding in ?^d is complete for the Existential Theory of the Reals for all d ? 3 and k ? {d-1,d}. Consequently, the problem is polynomial time equivalent to determining whether a polynomial equation system has a real solution and other important problems from various fields related to packing, Nash equilibria, minimum convex covers, the Art Gallery Problem, continuous constraint satisfaction problems, and training neural networks. Moreover, this implies NP-hardness and constitutes the first hardness result for the algorithmic problem of geometric embedding (abstract simplicial) complexes. This complements recent breakthroughs for the computational complexity of piece-wise linear embeddability
Rainbow Cycles in Flip Graphs
The flip graph of triangulations has as vertices all triangulations of a convex n-gon, and an edge between any two triangulations that differ in exactly one edge. An r-rainbow cycle in this graph is a cycle in which every inner edge of the triangulation appears exactly r times. This notion of a rainbow cycle extends in a natural way to other flip graphs. In this paper we investigate the existence of r-rainbow cycles for three different flip graphs on classes of geometric objects: the aforementioned flip graph of triangulations of a convex n-gon, the flip graph of plane spanning trees on an arbitrary set of n points, and the flip graph of non-crossing perfect matchings on a set of n points in convex position. In addition, we consider two flip graphs on classes of non-geometric objects: the flip graph of permutations of {1,2,...,n } and the flip graph of k-element subsets of {1,2,...,n }. In each of the five settings, we prove the existence and non-existence of rainbow cycles for different values of r, n and k
Rainbow cycles in flip graphs
The flip graph of triangulations has as vertices all triangulations of a convex -gon, and an edge between any two triangulations that differ in exactly one edge. An -rainbow cycle in this graph is a cycle in which every inner edge of the triangulation appears exactly ~times. This notion of a rainbow cycle extends in a natural way to other flip graphs. In this paper we investigate the existence of -rainbow cycles for three different flip graphs on classes of geometric objects: the aforementioned flip graph of triangulations of a convex -gon, the flip graph of plane trees on an arbitrary set of ~points, and the flip graph of non-crossing perfect matchings on a set of ~points in convex position. In addition, we consider two flip graphs on classes of non-geometric objects: the flip graph of permutations of and the flip graph of -element subsets of . In each of the five settings, we prove the existence and non-existence of rainbow cycles for different values of~, and~
Scheduling with Machine Conflicts
We study the scheduling problem of makespan minimization while taking machine
conflicts into account. Machine conflicts arise in various settings, e.g.,
shared resources for pre- and post-processing of tasks or spatial restrictions.
In this context, each job has a blocking time before and after its processing
time, i.e., three parameters. We seek for conflict-free schedules in which the
blocking times of no two jobs intersect on conflicting machines. Given a set of
jobs, a set of machines, and a graph representing machine conflicts, the
problem SchedulingWithMachineConflicts (SMC), asks for a conflict-free schedule
of minimum makespan.
We show that, unless , SMC on machines does not
allow for a -approximation algorithm for any
, even in the case of identical jobs and every choice of fixed
positive parameters, including the unit case. Complementary, we provide
approximation algorithms when a suitable collection of independent sets is
given. Finally, we present polynomial time algorithms to solve the problem for
the case of unit jobs on special graph classes. Most prominently, we solve it
for bipartite graphs by using structural insights for conflict graphs of star
forests.Comment: 20 pages, 8 figure
Packing Squares into a Disk with Optimal Worst-Case Density
We provide a tight result for a fundamental problem arising from packing squares into a circular container: The critical density of packing squares into a disk is ? = 8/(5?)? 0.509. This implies that any set of (not necessarily equal) squares of total area A ? 8/5 can always be packed into a disk with radius 1; in contrast, for any ? > 0 there are sets of squares of total area 8/5+? that cannot be packed, even if squares may be rotated. This settles the last (and arguably, most elusive) case of packing circular or square objects into a circular or square container: The critical densities for squares in a square (1/2), circles in a square (?/(3+2?2) ? 0.539) and circles in a circle (1/2) have already been established, making use of recursive subdivisions of a square container into pieces bounded by straight lines, or the ability to use recursive arguments based on similarity of objects and container; neither of these approaches can be applied when packing squares into a circular container. Our proof uses a careful manual analysis, complemented by a computer-assisted part that is based on interval arithmetic. Beyond the basic mathematical importance, our result is also useful as a blackbox lemma for the analysis of recursive packing algorithms. At the same time, our approach showcases the power of a general framework for computer-assisted proofs, based on interval arithmetic
Adjacency Graphs of Polyhedral Surfaces
We study whether a given graph can be realized as an adjacency graph of the
polygonal cells of a polyhedral surface in . We show that every
graph is realizable as a polyhedral surface with arbitrary polygonal cells, and
that this is not true if we require the cells to be convex. In particular, if
the given graph contains , , or any nonplanar -tree as a
subgraph, no such realization exists. On the other hand, all planar graphs,
, and can be realized with convex cells. The same holds for
any subdivision of any graph where each edge is subdivided at least once, and,
by a result from McMullen et al. (1983), for any hypercube.
Our results have implications on the maximum density of graphs describing
polyhedral surfaces with convex cells: The realizability of hypercubes shows
that the maximum number of edges over all realizable -vertex graphs is in
. From the non-realizability of , we obtain that
any realizable -vertex graph has edges. As such, these graphs
can be considerably denser than planar graphs, but not arbitrarily dense.Comment: To appear in Proc. SoCG 202
The Complexity of Recognizing Geometric Hypergraphs
As set systems, hypergraphs are omnipresent and have various representations
ranging from Euler and Venn diagrams to contact representations. In a geometric
representation of a hypergraph , each vertex is associated
with a point and each hyperedge is associated
with a connected set such that for all . We say that a given
hypergraph is representable by some (infinite) family of sets in
, if there exist and such
that is a geometric representation of . For a family F, we define
RECOGNITION(F) as the problem to determine if a given hypergraph is
representable by F. It is known that the RECOGNITION problem is
-hard for halfspaces in . We study the
families of translates of balls and ellipsoids in , as well as of
other convex sets, and show that their RECOGNITION problems are also
-complete. This means that these recognition problems are
equivalent to deciding whether a multivariate system of polynomial equations
with integer coefficients has a real solution.Comment: Appears in the Proceedings of the 31st International Symposium on
Graph Drawing and Network Visualization (GD 2023) 17 pages, 11 figure