86 research outputs found
Faster Fr\'echet Distance Approximation through Truncated Smoothing
The Fr\'echet distance is a popular distance measure for curves. Computing
the Fr\'echet distance between two polygonal curves of vertices takes
roughly quadratic time, and conditional lower bounds suggest that even
approximating to within a factor cannot be done in strongly-subquadratic
time, even in one dimension. The current best approximation algorithms present
trade-offs between approximation quality and running time. Recently, van der
Horst (SODA, 2023) presented an
time -approximate algorithm for curves in arbitrary dimensions, for any
. Our main contribution is an approximation algorithm for
curves in one dimension, with a significantly faster running time of . Additionally, we give an
algorithm for curves in arbitrary dimensions that improves upon the
state-of-the-art running time by a logarithmic factor, to . Both of our algorithms rely on a linear-time simplification
procedure that in one dimension reduces the complexity of the reachable free
space to without making sacrifices in the asymptotic
approximation factor.Comment: 27 pages, 11 figure
Chromatic k-Nearest Neighbor Queries
Let be a set of colored points. We develop efficient data structures
that store and can answer chromatic -nearest neighbor (-NN) queries.
Such a query consists of a query point and a number , and asks for the
color that appears most frequently among the points in closest to .
Answering such queries efficiently is the key to obtain fast -NN
classifiers. Our main aim is to obtain query times that are independent of
while using near-linear space.
We show that this is possible using a combination of two data structures. The
first data structure allow us to compute a region containing exactly the
-nearest neighbors of a query point , and the second data structure can
then report the most frequent color in such a region. This leads to linear
space data structures with query times of for points in
, and with query times varying between
and , depending on the distance measure used, for
points in . Since these query times are still fairly large we
also consider approximations. If we are allowed to report a color that appears
at least times, where is the frequency of the most
frequent color, we obtain a query time of in and expected query
times ranging between and
in using near-linear
space (ignoring polylogarithmic factors).Comment: 37 pages, 9 figure
Computing Minimum Complexity 1D Curve Simplifications under the Fréchet Distance
We consider the problem of simplifying curves under the Fréchet distance. Let P be a curve and Δ ℠0 be a distance threshold. An Δ-simplification is a curve within Fréchet distance Δ of P . We consider Δ-simplifications of minimum complexity (i.e. minimum number of vertices). Parameterized by Δ, we define a continuous family of minimum complexity Δ-simplifications P Δ of a curve P inone dimension. We present a data structure that after linear preprocessing time can report the Δ-simplification in linear output-sensitive time. Moreover, for k ℠1, we show how this data structure can be used to report a simplification P Δ with at most k vertices that is closest to P in O(k) time
Simply Realising an Imprecise Polyline is NP-hard
We consider the problem of deciding, given a sequence of regions, if there is a choice of points, one for each region, such that the induced polyline is simple or weakly simple, meaning that it can touch but not cross itself. Specifically, we consider the case where each region is a translate of the same shape. We show that the problem is NP-hard when the shape is a unit-disk or unit-square. We argue that the problem is is NP-complete when the shape is a vertical unit-segment
Faster and Deterministic Subtrajectory Clustering
We study the subtrajectory clustering problem. Given a trajectory , the goal is to identify a set of subtrajectories such that each point on is included in at least one subtrajectory, and subsequently group these subtrajectories together based on similarity under the FrĂ©chet distance. We wish to minimize the set of groups. This problem was shown to be NP-complete by Akitaya, BrĂŒning, Chambers, and Driemel (2021), and the focus has mainly been on approximation algorithms. We study a restricted variant, where we may only pick subtrajectories that start and end at vertices of , and give an approximation algorithm that significantly improves previous algorithms in both running time and space, whilst being deterministic
Robust Bichromatic Classification using Two Lines
Given two sets and of at most points
in the plane, we present efficient algorithms to find a two-line linear
classifier that best separates the "red" points in from the "blue"
points in and is robust to outliers. More precisely, we find a region
bounded by two lines, so either a halfplane, strip,
wedge, or double wedge, containing (most of) the blue points , and
few red points. Our running times vary between optimal and
, depending on the type of region and whether
we wish to minimize only red outliers, only blue outliers, or both.Comment: 19 pages, 11 figures. Updated to include new result
A Subquadratic nΔ-approximation for the Continuous Fréchet Distance
The FrĂ©chet distance is a commonly used similarity measure between curves. It is known how to compute the continuous FrĂ©chet distance between two polylines with m and n vertices in R^d in O(mn(log log n)ÂČ) time; doing so in strongly subquadratic time is a longstanding open problem. Recent conditional lower bounds suggest that it is unlikely that a strongly subquadratic algorithm exists. Moreover, it is unlikely that we can approximate the Fr Ìechet distance to within a factor 3 in strongly subquadratic time, even if d = 1. The best current results establish a tradeoff between approximation quality and running time. Specifically, Colombe and Fox (SoCG, 2021) give an O(α)-approximate algorithm that runs in O((n3/α2) log n) time for any α â [ân, n], assuming m = n. In this paper, we improve this result with an O(α)-approximate algorithm that runs in O((n + mn/α) logÂł n) time for any α â [1, n], assuming m †n and constant dimension d
Variables associated with in-hospital and postdischarge outcomes after postcardiotomy extracorporeal membrane oxygenation:Netherlands Heart Registration Cohort
Objectives: Extracorporeal membrane oxygenation (ECMO) for postcardiotomy cardiogenic shock has been increasingly used without concomitant mortality reduction. This study aims to investigate determinants of in-hospital and postdischarge mortality in patients requiring postcardiotomy ECMO in the Netherlands. Methods: The Netherlands Heart Registration collects nationwide prospective data from cardiac surgery units. Adults receiving intraoperative or postoperative ECMO included in the register from January 2013 to December 2019 were studied. Survival status was established through the national Personal Records Database. Multivariable logistic regression analyses were used to investigate determinants of in-hospital (3 models) and 12-month postdischarge mortality (4 models). Each model was developed to target specific time points during a patient's clinical course. Results: Overall, 406 patients (67.2% men, median age, 66.0 years [interquartile range, 55.0-72.0 years]) were included. In-hospital mortality was 51.7%, with death occurring in a median of 5 days (interquartile range, 2-14 days) after surgery. Hospital survivors (n = 196) experienced considerable rates of pulmonary infections, respiratory failure, arrhythmias, and deep sternal wound infections during a hospitalization of median 29 days (interquartile range, 17-51 days). Older age (odds ratio [OR], 1.02; 95% CI, 1.0-1.04) and preoperative higher body mass index (OR, 1.08; 95% CI, 1.02-1.14) were associated with in-hospital death. Within 12 months after discharge, 35.1% of hospital survivors (n = 63) died. Postoperative renal failure (OR, 2.3; 95% CI, 1.6-4.9), respiratory failure (OR, 3.6; 95% CI, 1.3-9.9), and re-thoracotomy (OR, 2.9; 95% CI, 1.3-6.5) were associated with 12-month postdischarge mortality. Conclusions: In-hospital and postdischarge mortality after postcardiotomy ECMO in adults remains high in the Netherlands. ECMO support in patients with higher age and body mass index, which drive associations with higher in-hospital mortality, should be carefully considered. Further observations suggest that prevention of re-thoracotomies, renal failure, and respiratory failure are targets that may improve postdischarge outcomes
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