scholarly journals Efficient computation of optimal temporal walks under waiting-time constraints

2020 ◽  
Vol 5 (1) ◽  
Author(s):  
Matthias Bentert ◽  
Anne-Sophie Himmel ◽  
André Nichterlein ◽  
Rolf Niedermeier
Keyword(s):  
Waiting Time ◽  
Experimental Studies ◽  
Time Constraints ◽  
Efficient Computation ◽  
Time Step ◽  
Optimization Criteria ◽  
Node Connectivity ◽  
Temporal Aspect ◽  
Temporal Graphs ◽  
Vertex Sets

Abstract Node connectivity plays a central role in temporal network analysis. We provide a broad study of various concepts of walks in temporal graphs, that is, graphs with fixed vertex sets but arc sets changing over time. Taking into account the temporal aspect leads to a rich set of optimization criteria for “shortest” walks. Extending and broadening state-of-the-art work of Wu et al. [IEEE TKDE 2016], we provide an algorithm for computing shortest walks that is capable to deal with various optimization criteria and any linear combination of these. It runs in O(|V|+|E|log|E|) time where |V| is the number of vertices and |E| is the number of time-arcs. A central distinguishing factor to Wu et al.’s work is that our model allows to, motivated by real-world applications, respect waiting-time constraints for vertices, that is, the minimum and maximum waiting time allowed in intermediate vertices of a walk. Moreover, other than Wu et al. our algorithm also allows to search for walks that pass multiple subsequent time-arcs in one time step, and it can deal with a richer set of optimization criteria. Our experimental studies indicate that our richer modeling can be achieved without significantly worsening the running time when compared to Wu et al.’s algorithms.

Multi-Head Spatio-Temporal Attention Mechanism for Urban Anomaly Event Prediction

2021 ◽  
Vol 5 (3) ◽  
pp. 1-21
Author(s):  
Huiqun Huang ◽  
Xi Yang ◽  
Suining He
Keyword(s):  
New York ◽  
Experimental Studies ◽  
Attention Mechanism ◽  
City Management ◽  
Temporal Attention ◽  
Spatial Correlations ◽  
Time Step ◽  
Event Prediction ◽  
Spatio Temporal ◽  
Prediction Approach

Timely forecasting the urban anomaly events in advance is of great importance to the city management and planning. However, anomaly event prediction is highly challenging due to the sparseness of data, geographic heterogeneity (e.g., complex spatial correlation, skewed spatial distribution of anomaly events and crowd flows), and the dynamic temporal dependencies. In this study, we propose M-STAP, a novel Multi-head Spatio-Temporal Attention Prediction approach to address the problem of multi-region urban anomaly event prediction. Specifically, M-STAP considers the problem from three main aspects: (1) extracting the spatial characteristics of the anomaly events in different regions, and the spatial correlations between anomaly events and crowd flows; (2) modeling the impacts of crowd flow dynamic of the most relevant regions in each time step on the anomaly events; and (3) employing attention mechanism to analyze the varying impacts of the historical anomaly events on the predicted data. We have conducted extensive experimental studies on the crowd flows and anomaly events data of New York City, Melbourne and Chicago. Our proposed model shows higher accuracy (41.91% improvement on average) in predicting multi-region anomaly events compared with the state-of-the-arts.

scholarly journals Optimizing Reachability Sets in Temporal Graphs by Delaying

2020 ◽  
Vol 34 (06) ◽  
pp. 9810-9817
Author(s):  
Argyrios Deligkas ◽  
Igor Potapov
Keyword(s):  
Polynomial Time Algorithm ◽  
Time Algorithm ◽  
Control Mechanisms ◽  
Scheduling Problems ◽  
Dynamic Graph ◽  
Time Step ◽  
Supply Networks ◽  
Reachability Sets ◽  
Network Epidemiology ◽  
Temporal Graphs

A temporal graph is a dynamic graph where every edge is assigned a set of integer time labels that indicate at which discrete time step the edge is available. In this paper, we study how changes of the time labels, corresponding to delays on the availability of the edges, affect the reachability sets from given sources. The questions about reachability sets are motivated by numerous applications of temporal graphs in network epidemiology and scheduling problems in supply networks in manufacturing. We introduce control mechanisms for reachability sets that are based on two natural operations of delaying time events. The first operation, termed merging, is global and batches together consecutive time labels in the whole network simultaneously. This corresponds to postponing all events until a particular time. The second, imposes independent delays on the time labels of every edge of the graph. We provide a thorough investigation of the computational complexity of different objectives related to reachability sets when these operations are used. For the merging operation, we prove NP-hardness results for several minimization and maximization reachability objectives, even for very simple graph structures. For the second operation, we prove that the minimization problems are NP-hard when the number of allowed delays is bounded. We complement this with a polynomial-time algorithm for the case of unbounded delays.

scholarly journals Time constraints on experimental studies of lead apatites

2019 ◽  
Author(s):  
Justyna TOPOLSKA ◽  
Tomasz BAJDA ◽  
Bartosz PUZIO ◽  
Maciej MANECKI ◽  
Gabriela KOZUB-BUDZYŃ KOZUB-BUDZYŃ
Keyword(s):  
Experimental Studies ◽  
Time Constraints

Optimal Stochastic Allocation of Machines under Waiting-Time Constraints

1993 ◽  
Vol 22 (2) ◽  
pp. 332-348 ◽  
Author(s):  
E. G. Coffman, Jr. ◽  
Leopold Flatto ◽  
Paul E. Wright
Keyword(s):  
Waiting Time ◽  
Time Constraints

Flow Field and Vibration Behavior of Straight Transonic Compressor Cascade due to External Acoustic Excitation

2013 ◽  
Author(s):  
Manlu Li ◽  
Anping Hou ◽  
Xiaodong Yang ◽  
Mingming Zhang ◽  
Peng Wang
Keyword(s):  
Flow Field ◽  
Excitation Frequency ◽  
Experimental Studies ◽  
Attack Angle ◽  
Mode Shapes ◽  
Acoustic Excitation ◽  
Fe Model ◽  
Compressor Cascade ◽  
Time Step ◽  
Vibration Behavior

A fluid-structure coupled approach is utilized to study the influence of external acoustic excitation on straight compressor cascade flow field and blade vibration behavior. Interaction between fluid and structure are dealt with in a coupled manner, based on the interface exchange of information between the aerodynamic and structural model. The computation fluid mesh is updated at every time step with an improved algebraic method. The flow field of cascade with/without external acoustic excitation is carried out using a 3D unsteady CFD model based on moving boundary way, as well as some experimental studies based on transonic wind tunnel. Then coupled with blade FE model, mode shapes, frequencies, vibration stress and the structural deformations of blade are identified. The performance of the cascade is obtained by computational and experimental ways, consistency of numerical and test results shows that the numerical model is suitable. The numerical results show that acoustic excitation has a greater impact on negative and designed attack angle in contrast to high positive attack angle. The cascade wake and blade surface pressure frequency characteristic are changed and the main frequency is almost the same as the acoustic excitation frequency. Compared results with no excitation, the vibration characteristics of the blade is changed, also the vibration behavior is sensitive to the excitation amplitude and frequency.

Stable Communities Detection Method for Temporal Multiplex Graphs: Heterogeneous Social Network Case Study

2020 ◽  
Author(s):  
Wala Rebhi ◽  
Nesrine Ben Yahia ◽  
Narjès Bellamine Ben Saoud
Keyword(s):  
Social Network ◽  
Community Detection ◽  
Detection Algorithm ◽  
Step Method ◽  
Temporal Dimension ◽  
Temporal Aspect ◽  
Heterogeneous Social Networks ◽  
Community Detection Algorithm ◽  
High Level ◽  
Temporal Graphs

Abstract Multiplex graphs have been recently proposed as a model to represent high-level complexity in real-world networks such as heterogeneous social networks where actors could be characterized by heterogeneous properties and could be linked with different types of social interactions. This has brought new challenges in community detection, which aims to identify pertinent groups of nodes in a complex graph. In this context, great efforts have been made to tackle the problem of community detection in multiplex graphs. However, most of the proposed methods until recently deal with static multiplex graph and ignore the temporal dimension, which is a key characteristic of real networks. Even more, the few methods that consider temporal graphs, they just propose to follow communities over time and none of them use the temporal aspect directly to detect stable communities, which are often more meaningful in reality. Thus, this paper proposes a new two-step method to detect stable communities in temporal multiplex graphs. The first step aims to find the best static graph partition at each instant by applying a new hybrid community detection algorithm, which considers both relations heterogeneities and nodes similarities. Then, the second step considers the temporal dimension in order to find final stable communities. Finally, experiments on synthetic graphs and a real social network show that this method is competitive and it is able to extract high-quality communities.

Simulation of Red Blood Cell Passing Through Constrictions Using Modified Moving Particle Semi-Implicit Method

2011 ◽  
Author(s):  
K. Firoozbakhsh ◽  
M. T. Ahmadian ◽  
M. Hasanian
Keyword(s):  
Mechanical Behavior ◽  
Experimental Studies ◽  
Small Time ◽  
Implicit Method ◽  
Parallel Plates ◽  
Time Step ◽  
Mps Method ◽  
Time Step Size ◽  
Small Time Step ◽  
Moving Particle

During the circulation of RBC it undergoes elastic deformation as it passes through micro-capillaries where the inner diameter of the constriction can be about 3 micro meters. It means RBC shape must be changed in order to pass through these narrow channels. The role of mechanical behavior of RBC and the deformability traits of RBC are observed with the several experimental studies [1]. Several methods were implemented to simulate the mechanical behavior of RBCs in micro-capillaries [1, 2]. One of the most recent methods is Moving Particle Semi-implicit method (MPS) which is a Lagrangian method with semi-implicit algorithm that guaranties the incompressibility of the fluid. MPS method was implemented for simulation of RBC motion through parallel plates by Tsubota et al. 2006 [3]. Due to small Reynolds number and the Diffusion number restrictions, implementation of small time step size would be necessary which leads to long time simulation. By the way in case of complex geometries or FSI problems, standard MPS method has a delicate pressure solver which leads to diverge the solution. So in these cases using a small time step can help to overcome the problem. Some studies have applied a new approach for time integration and the fractional time step method is employed to overcome the noticed problem. Yohsuke Imai and coworkers (2010) have developed the former studies with two main new approaches [4]. Firstly, evaluation of viscosity is upgraded and secondly boundary condition is assumed to be periodic. Although the developments are really impressive and MPS method has turned into a practical method for simulation of RBC motion in micro-capillaries, but still there are some considerations about using large time steps and error of the velocity profile consequently.

Multi-item spare parts systems with lateral transshipments and waiting time constraints

2006 ◽  
Vol 171 (3) ◽  
pp. 1071-1093 ◽  
Author(s):  
H. Wong ◽  
G.J. van Houtum ◽  
D. Cattrysse ◽  
D. Van Oudheusden
Keyword(s):  
Waiting Time ◽  
Spare Parts ◽  
Time Constraints
Sign in / Sign up

Export Citation Format

Share Document