337,580 research outputs found
Bringing Order to Special Cases of Klee's Measure Problem
Klee's Measure Problem (KMP) asks for the volume of the union of n
axis-aligned boxes in d-space. Omitting logarithmic factors, the best algorithm
has runtime O*(n^{d/2}) [Overmars,Yap'91]. There are faster algorithms known
for several special cases: Cube-KMP (where all boxes are cubes), Unitcube-KMP
(where all boxes are cubes of equal side length), Hypervolume (where all boxes
share a vertex), and k-Grounded (where the projection onto the first k
dimensions is a Hypervolume instance).
In this paper we bring some order to these special cases by providing
reductions among them. In addition to the trivial inclusions, we establish
Hypervolume as the easiest of these special cases, and show that the runtimes
of Unitcube-KMP and Cube-KMP are polynomially related. More importantly, we
show that any algorithm for one of the special cases with runtime T(n,d)
implies an algorithm for the general case with runtime T(n,2d), yielding the
first non-trivial relation between KMP and its special cases. This allows to
transfer W[1]-hardness of KMP to all special cases, proving that no n^{o(d)}
algorithm exists for any of the special cases under reasonable complexity
theoretic assumptions. Furthermore, assuming that there is no improved
algorithm for the general case of KMP (no algorithm with runtime O(n^{d/2 -
eps})) this reduction shows that there is no algorithm with runtime
O(n^{floor(d/2)/2 - eps}) for any of the special cases. Under the same
assumption we show a tight lower bound for a recent algorithm for 2-Grounded
[Yildiz,Suri'12].Comment: 17 page
On distance measures for well-distributed sets
In this paper we investigate the Erd\"os/Falconer distance conjecture for a
natural class of sets statistically, though not necessarily arithmetically,
similar to a lattice. We prove a good upper bound for spherical means that have
been classically used to study this problem. We conjecture that a majorant for
the spherical means suffices to prove the distance conjecture(s) in this
setting. For a class of non-Euclidean distances, we show that this generally
cannot be achieved, at least in dimension two, by considering integer point
distributions on convex curves and surfaces. In higher dimensions, we link this
problem to the question about the existence of smooth well-curved hypersurfaces
that support many integer points
An improved bound on the number of point-surface incidences in three dimensions
We show that points and smooth algebraic surfaces of bounded degree
in satisfying suitable nondegeneracy conditions can have at most
incidences, provided that any
collection of points have at most O(1) surfaces passing through all of
them, for some . In the case where the surfaces are spheres and no
three spheres meet in a common circle, this implies there are point-sphere incidences. This is a slight improvement over the previous
bound of for an (explicit) very
slowly growing function. We obtain this bound by using the discrete polynomial
ham sandwich theorem to cut into open cells adapted to the set
of points, and within each cell of the decomposition we apply a Turan-type
theorem to obtain crude control on the number of point-surface incidences. We
then perform a second polynomial ham sandwich decomposition on the irreducible
components of the variety defined by the first decomposition. As an
application, we obtain a new bound on the maximum number of unit distances
amongst points in .Comment: 17 pages, revised based on referee comment
Computational Geometry Column 42
A compendium of thirty previously published open problems in computational
geometry is presented.Comment: 7 pages; 72 reference
Far-Field Compression for Fast Kernel Summation Methods in High Dimensions
We consider fast kernel summations in high dimensions: given a large set of
points in dimensions (with ) and a pair-potential function (the
{\em kernel} function), we compute a weighted sum of all pairwise kernel
interactions for each point in the set. Direct summation is equivalent to a
(dense) matrix-vector multiplication and scales quadratically with the number
of points. Fast kernel summation algorithms reduce this cost to log-linear or
linear complexity.
Treecodes and Fast Multipole Methods (FMMs) deliver tremendous speedups by
constructing approximate representations of interactions of points that are far
from each other. In algebraic terms, these representations correspond to
low-rank approximations of blocks of the overall interaction matrix. Existing
approaches require an excessive number of kernel evaluations with increasing
and number of points in the dataset.
To address this issue, we use a randomized algebraic approach in which we
first sample the rows of a block and then construct its approximate, low-rank
interpolative decomposition. We examine the feasibility of this approach
theoretically and experimentally. We provide a new theoretical result showing a
tighter bound on the reconstruction error from uniformly sampling rows than the
existing state-of-the-art. We demonstrate that our sampling approach is
competitive with existing (but prohibitively expensive) methods from the
literature. We also construct kernel matrices for the Laplacian, Gaussian, and
polynomial kernels -- all commonly used in physics and data analysis. We
explore the numerical properties of blocks of these matrices, and show that
they are amenable to our approach. Depending on the data set, our randomized
algorithm can successfully compute low rank approximations in high dimensions.
We report results for data sets with ambient dimensions from four to 1,000.Comment: 43 pages, 21 figure
- …