Activity Levels at Hub Facilities in Interacting Networks

Hubs are a special type of central facility which are designed to act as switching points for intemodal flows. For instance, a set of ten interacting cities might all be connected to one of two major hubs. All flows between the cities would then be routed via the hubs. There is an obvious saving in the number of routes necessary to interconnect the cities when hubs are utilized, with a concomitant high level of activity at the facilities. This paper takes a heuristic approach to the evaluation of networks and hub locations to find locally optimal designs. It is shown that minimization of transportation costs may require assignment of nodes to a facility other than the nearest. A discount on the interhub transportation costs promotes a wider spacing of facilities. In a system with several hubs, minimization of total hub usage tends to concentrate demand very heavily into one central facility.     

Testing Policy Theory with Statistical Models of Networks

This article presents a conceptual framework for clarifying the network hypotheses embedded in policy theories and how they relate to macrolevel political institutions and microlevel political behavior. We then describe the role of statistical models of networks for testing these hypotheses, including the problem of operationalizing theoretical concepts with the parameters of statistical models. Examples from existing theories of the policy process and empirical research are provided and potential extensions are discussed.     

Spatial Characteristics and Comparisons of Interaction and Median Clustering Models

Cluster analysis has been pursued from a number of directions for identifying interesting relationships and patterns in spatial information. A major emphasis is currently on the development and refinement of optimization‐based clustering models for the purpose of exploring spatially referenced data. Within this context, two basic methods exist for identifying clusters that are most similar. An interesting feature of these two approaches is that one method approximates the relationships inherent in the other method. This is significant given that the approximation approach is invariably utilized for cluster detection in spatial and aspatial analysis. A number of spatial applications are investigated which highlight the differences in clusters produced by each model. This is an important contribution because the differences are in fact quite significant, yet these contrasts are not widely known or acknowledged.     

A Note on Trip Benefits in Spatial Interaction Models

We extend the well-known transport users' benefits measure (TUB) for the doubly-constrained spatial interaction model derived by Williams (1976). The original formula expresses the TUB as composed by two terms associated with the origin and the destination zones. First, the TUB is associated here with trips instead of zones, providing a natural interpretation as a rule-of-a-half measure of benefit under inelastic demand (for the short-run case). Second, a TUB formula for the long-run case is derived, that is, when the total number of trips, trip origins, and trip destinations change. We then propose updated measures of accessibility for location behavior.     

Dynamic Models of Agricultural Location in a Spatial Interaction Framework

Recent research demonstrates that spatial interaction models may also be made to function as location models by the addition of appropriate hypotheses about structural adjustment. An appealing feature of the approach is that dynamics are explicitly incorporated. In this paper, the attempt is made to recast a problem from classical economic-geographic theory—that of agricultural location—within a spatial interaction framework. It is shown that a wide variety of models is potentially available, and the properties of a range of these models are selectively explored.     

Using Simple Genetic Algorithms to Calibrate Spatial Interaction Models

The paper investigates the use of two genetic algorithms in an attempt to obtain globally optimal parameter estimates for a mix of simple and complex spatial interaction models. The genetic algorithms work well and are strongly advocated as a more robust approach particularly for use with the more complex multiparameter models where the differences in both performance and parameter values are judged to be significant.     

Explorations and Syntheses of Linear Programming And Spatial Interaction Models of Residential Location

Various “linear programming” models of residential location are explored in some detail. It is then shown that entropy maximizing “sub-optimal” versions of these can be developed, which may be more realistic at typical levels of resolution used for analysis but can still be given some of the interpretations of the programming models. Further, any programming model can be seen as a limiting case of some entropy maximizing model in which some of the parameters tend to infinity. Some empirical tests of both kinds of models are presented, and the limiting relationships are discussed.     

Analysis of Dynamic Spatial Interaction Models by Means of Optimal Control

This paper deals with the design of general classes of dynamic spatial interaction models. On the basis of a general (well-behaved) multiperiod objective function and of a dynamic model representing the evolution of a spatial interaction system, an optimal control model is constructed. Particular attention is given to the equilibrium and stability conditions. It turns out that it is possible to identify steady-state solutions for a dynamic spatial interaction model. Furthermore, it can also be demonstrated that the entropy model is a specific case of this spatial interaction system. A simple illustration for urban dynamics is given as well.     

Learning in Single Hidden-Layer Feedforward Network Models: Backpropagation in a Spatial Interaction Modeling Context

Learning in neural networks has attracted considerable interest in recent years. Our focus is on learning in single hidden-layer feedforward networks which is posed as a search in the network parameter space for a network that minimizes an additive error function of statistically independent examples. We review first the class of single hidden-layer feedforward networks and characterize the learning process in such networks from a statistical point of view. Then we describe the backpropagation procedure, the leading case of gradient descent learning algorithms for the class of networks considered here, as well as an efficient heuristic modification. Finally, we analyze the applicability of these learning methods to the problem of predicting interregional telecommunication flows. Particular emphasis is laid on the engineering judgment, first, in choosing appropriate values for the tunable parameters, second, on the decision whether to train the network by epoch or by pattern (random approximation), and, third, on the overfitting problem. In addition, the analysis shows that the neural network model whether using either epoch-based or pattern-based stochastic approximation outperforms the classical regression approach to modeling telecommunication flows.     

Building an Automated Modeling System to Explore a Universe of Spatial Interaction Models

A methodology is described for harnessing the power of supercomputer hardware to build an automated modeling system. The problems of applying this new approach to mathematical modeling in geography are discussed by reference to the design of better performing spatial interaction models. It is also possible that the results may provide clues about new theories and knowledge.     

Micro‐Level Interpretation of Exponential Random Graph Models with Application to Estuary Networks

The exponential random graph model (ERGM) is an increasingly popular method for the statistical analysis of networks that can be used to flexibly analyze the processes by which policy actors organize into a network. Often times, interpretation of ERGM results is conducted at the network level, such that effects are related to overall frequencies of network structures (e.g., the number of closed triangles in a network). This limits the utility of the ERGM because there is often interest, particularly in political and policy sciences, in network dynamics at the actor or relationship levels. Micro‐level interpretation of the ERGM has been employed in varied applications in sociology and statistics. We present a comprehensive framework for interpretation of the ERGM at all levels of analysis, which casts network formation as block‐wise updating of a network. These blocks can represent, for example, each potential link, each dyad, the out‐ or in‐going ties of each actor, or the entire network. We contrast this interpretive framework with the stochastic actor‐based model (SABM) of network dynamics. We present the theoretical differences between the ERGM and the SABM and introduce an approach to comparing the models when theory is not sufficiently strong to make the selection a priori. The alternative models we discuss and the interpretation methods we propose are illustrated on previously published data on estuary policy and governance networks.     

Extensions to the Hub Location Problem: Formulations and Numerical Examples

Although a tremendous amount of analytical research is being conducted on the hub location problem, few models exist that extend the number of characteristics found in actual hub-and-spoke networks. Four extensions are presented in this paper: (1) a capacitated network model; (2) a minimum threshold model; (3) a model that endogenously determines the number of open hubs for the network; and (4) a model that incorporates a flow-dependent cost function for the spokes as well as the interhub links. Both the capacitated and the minimum threshold models drop the assumption of a completely interconnected network commonly found in hub location models. Numerical results show that total network costs are often minimized by closing a few interhub links. The third extension is the first known hub location model to determine the optimal number of hubs based on the needs of the network. In this model, the number of open hubs depends on the distribution of flows in the network and how cost effectively the flows can be moved across the network. Previous models that endogenously determined the number of open hubs utilized a fixed cost for establishing each hub in order to limit the number of hubs in the network. The final extension recognizes the potential of all links to amalgamate flows and includes a separate flow-dependent cost function for the spokes in addition to the one for the interhub links. Numerical results are shown for all four models.     

New Perspectives on Early Regional Interaction Networks of East African Trade: A View from Tsavo National Park,Kenya

Archaeologists are interested in understanding whether cross-cultural contact catalyzed by exchange of commodities is a means for people to acquire new technology and cultural ideas. This paper reports the results and analysis of archaeological investigations in Tsavo National Park, Kenya in 2001 and 2004 that have recovered evidence of indirect contact between late pastoral neolithic (PN) herders and early iron working (EIW) communities. Analysis of stylistic motifs on ceramics used by both PN and EIW people show that socially constructed notions of aesthetics are similar, implying that a strong cultural connection between the two groups may be possible. New lexico-statistical analyses and archaeological data from Tanzania corroborate that the prehistoric East African milieu was more complex than previously believed. However, pastoralists in Tsavo show no evidence of utilizing iron technology or altering their subsistence strategies among the > 10,000 artifacts spanning 4,000+ years of prehistory recovered in the 2001 field season. Thus, long-distance and long-term contact between socially connected groups possessing different technological and subsistence foundations does not necessarily lead to a diffusion of new survival strategies.Les archéologues sont intéressés à comprendre la diffusion technologique et culturelle préhistorique [Les archeologues se sont interestesa comprendre la diffusion prehistorique de la technologie et de la culture]. Le contact entre les cultures catalysées par l'échange des produits est des moyens pour que les personnes acquièrent la nouvelle technologie et les idées culturelles. Cependant, le point auquel certaines idées sont acceptées ou rejetées demeure irrésolu. [Quand le contact contre-culturelle s'allie avec l'exchange de produits, les deux fournissent les nouvelles idees culturelles et technologiques aux gens. Mais, le point d D'autres données sont nécessaires pour augmenter nos compréhension de l'échange des idées entre les groupes de personnes et comment des changements de technologie. Les investigations archéologiques en parc national de Tsavo, Kenya dans 2001 ont récupéré l'évidence du contact indirect entre les bouviers néolithiques pastoraux (PN) en retard et les premières communautés du fondre de fer (EIW). L'analyse des motifs stylistiques sur la céramique utiliser par des personnes de PN et d'EIW prouvent que les notions socialement construites de l'esthétique sont semblables, impliquant qu'un raccordement culturel fort entre les deux groupes peut être possible. Les nouvelles analyses lexico-statistiques et les données archéologiques de Tanzanie corroborent que le milieu africain est préhistorique était plus complexe que précédemment cru. Cependant, les bouviers préhistorique dans Tsavo ne montrent aucune évidence d'utiliser la technologie de fer ou de changer leurs stratégies de subsistance parmi les > 10.000 objets façonnés enjambant les années 4.000+ de la préhistoire récupérées dans la saison de 2001 champs. Ainsi, contact à longue distance et long terme entre les groupes socialement reliés possédant différentes bases technologiques et de subsistance ne mène pas nécessairement à une diffusion de nouvelles strategies de survival.     

华人网络与韩人网络的比较研究

通过当前韩国对于海外华人四个方面的"似是而非"认识的分析,对华商网络的倾向与特征、建设韩民族网络等问题的探讨,得出结论:韩国华侨与韩国所理解的海外华人之间存在很大的差异;当代韩国政府希望能像中国充分发挥海外华人的优势那样,使海外韩人发展成为韩国在21世纪的新的成长动力并加以利用的认识是一种错误的"乐观判断";海外华人对祖国的向心力,不只是来自爱国情,更重要的是其生存保障与利益的实现;中韩两国的侨务政策存在区别,中国致力于"护侨",而韩国的目光集中于"用侨"。