Detection of Core-Periphery Structure in Networks Using Spectral Methods and Geodesic Paths

We introduce several novel and computationally efficient methods for detecting "core--periphery structure" in networks. Core--periphery structure is a type of mesoscale structure that includes densely-connected core vertices and sparsely-connected peripheral vertices. Core vertices tend to be well-connected both among themselves and to peripheral vertices, which tend not to be well-connected to other vertices. Our first method, which is based on transportation in networks, aggregates information from many geodesic paths in a network and yields a score for each vertex that reflects the likelihood that a vertex is a core vertex. Our second method is based on a low-rank approximation of a network's adjacency matrix, which can often be expressed as a tensor-product matrix. Our third approach uses the bottom eigenvector of the random-walk Laplacian to infer a coreness score and a classification into core and peripheral vertices. We also design an objective function to (1) help classify vertices into core or peripheral vertices and (2) provide a goodness-of-fit criterion for classifications into core versus peripheral vertices. To examine the performance of our methods, we apply our algorithms to both synthetically-generated networks and a variety of networks constructed from real-world data sets.Comment: This article is part of EJAM's December 2016 special issue on "Network Analysis and Modelling" (available at https://www.cambridge.org/core/journals/european-journal-of-applied-mathematics/issue/journal-ejm-volume-27-issue-6/D245C89CABF55DBF573BB412F7651ADB

Cultural and symbolic capital in the market for security : police-private security relations in Mexico and the United Kingdom

Recent scholarship has observed how private security actors often draw upon the cultural and symbolic capital of the police in their everyday operations. This practice can range from issuing frontline private security officers with police-like uniforms and patrol cars to recruiting former senior police officers into highly visible corporate positions. Geographical variations in this dynamic are little understood, however. In this article, we identify and shed light upon one emergent pattern. In those countries where the police enjoy high levels of public trust and confidence, private security actors can be found openly and directly borrowing from the cultural and symbolic capital of this key state institution to enhance their status. By contrast, in those countries where the police are plagued by a poor reputation, these actors commonly display a far more ambiguous relationship with these forms of capital, working both through and against them, often at the same time. Focusing on the UK and Mexican cases, and drawing upon a combination of Bourdieusian frameworks, we argue that the key to understanding this pattern is the distinction between the objectified (non-human) cultural capital of the police (uniforms, vehicles, etc) and the embodied (human) cultural capital of the police (police officers themselves). While the former enjoys a symbolic value in the market for security which transcends variations in public trust and confidence in the police, the latter is far more intimately connected to localised police traditions and practices, good or bad. This in turn leads to novel patterns in the global plural policing landscape

Medial prefrontal cortex and hippocampal activity differentially contribute to ordinal and temporal context retrieval during sequence memory

Remembering sequences of events defines episodic memory, but retrieval can be driven by both ordinality and temporal contexts. Whether these modes of retrieval operate at the same time or not remains unclear. Theoretically, medial prefrontal cortex (mPFC) confers ordinality, while the hippocampus (HC) associates events in gradually changing temporal contexts. Here, we looked for evidence of each with BOLD fMRI in a sequence task that taxes both retrieval modes. To test ordinal modes, items were transferred between sequences but retained their position (e.g., AB3). Ordinal modes activated mPFC, but not HC. To test temporal contexts, we examined items that skipped ahead across lag distances (e.g., ABD). HC, but not mPFC, tracked temporal contexts. There was a mPFC and HC by retrieval mode interaction. These current results suggest that the mPFC and HC are concurrently engaged in different retrieval modes in support of remembering when an event occurred

Genetic Algorithms for the Discovery of Homogeneous Catalysts

In this account, we discuss the use of genetic algorithms in the inverse design process of homogeneous catalysts for chemical transformations. We describe the main components of evolutionary experiments, specifically the nature of the fitness function to optimize, the library of molecular fragments from which potential catalysts are assembled, and the settings of the genetic algorithm itself. While not exhaustive, this review summarizes the key challenges and characteristics of our own (i.e., NaviCatGA) and other GAs for the discovery of new catalysts