Large-scale structure of time evolving citation networks

In this paper we examine a number of methods for probing and understanding the large-scale structure of networks that evolve over time. We focus in particular on citation networks, networks of references between documents such as papers, patents, or court cases. We describe three different methods of analysis, one based on an expectation-maximization algorithm, one based on modularity optimization, and one based on eigenvector centrality. Using the network of citations between opinions of the United States Supreme Court as an example, we demonstrate how each of these methods can reveal significant structural divisions in the network, and how, ultimately, the combination of all three can help us develop a coherent overall picture of the network's shape.Comment: 10 pages, 6 figures; journal names for 4 references fixe

Deterministic Modularity Optimization

We study community structure of networks. We have developed a scheme for maximizing the modularity Q based on mean field methods. Further, we have defined a simple family of random networks with community structure; we understand the behavior of these networks analytically. Using these networks, we show how the mean field methods display better performance than previously known deterministic methods for optimization of Q.Comment: 7 pages, 4 figures, minor change

The pio Operon Is Essential for Phototrophic Fe(II) Oxidation in Rhodopseudomonas palustris TIE-1

Phototrophic Fe(II)-oxidizing bacteria couple the oxidation of ferrous iron [Fe(II)] to reductive CO2 fixation by using light energy, but until recently, little has been understood about the molecular basis for this process. Here we report the discovery, with Rhodopseudomonas palustris TIE-1 as a model organism, of a three-gene operon, designated the pio operon (for phototrophic iron oxidation), that is necessary for phototrophic Fe(II) oxidation. The first gene in the operon, pioA, encodes a c-type cytochrome that is upregulated under Fe(II)-grown conditions. PioA contains a signal sequence and shares homology with MtrA, a decaheme c-type cytochrome from Shewanella oneidensis MR-1. The second gene, pioB, encodes a putative outer membrane beta-barrel protein. PioB is a homologue of MtrB from S. oneidensis MR-1. The third gene, pioC, encodes a putative high potential iron sulfur protein (HiPIP) with a twin-arginine translocation (Tat) signal sequence and is similar to the putative Fe(II) oxidoreductase (Iro) from Acidithiobacillus ferrooxidans. Like PioA, PioB and PioC appear to be secreted proteins. Deletion of the pio operon results in loss of Fe(II) oxidation activity and growth on Fe(II). Complementation studies confirm that the phenotype of this mutant is due to loss of the pio genes. Deletion of pioA alone results in loss of almost all Fe(II) oxidation activity; however, deletion of either pioB or pioC alone results in only partial loss of Fe(II) oxidation activity. Together, these results suggest that proteins encoded by the pio operon are essential and specific for phototrophic Fe(II) oxidation in R. palustris TIE-1

Introduction

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Changing patterns of transition from school to university mathematics

There has been widespread concern over the lack of preparedness of students making the transition from school to university mathematics and the changing profile of entrants to mathematical subjects in higher education has been well documented. In this paper, using documentary analysis and data from an informal case study, we argue the antecedents of this changed profile in the general shift across all subjects to a more utilitarian higher education, alongside the more specific changes in A-level mathematics provision which have been largely market driven. Our conclusions suggest that, ironically, changes put in place to make mathematics more widely useful may result in it losing just those features that make it marketable

How effective was the drone campaign in Pakistan, Yemen, and Somalia throughout Barack Obama’s presidency, 2009 – 2017?

This research analyses the failings and the overall effectiveness of the United States drone campaign operating in Pakistan, Yemen, and Somalia throughout Barack Obama’s Presidency, 2009 - 2017. Effective being defined as how successful the US has been in targeting individuals who pose a legitimate threat to the US and its interests. The debate around US drone strikes is somewhat silenced, due to the intense secrecy surrounding many aspects of the programme. However, the proliferation of strikes under Obama has meant that more scholars and journalists are questioning methods used by the Obama administration to target individuals. This means that only recently have more articles and books been published specifically relating to these practises. This study will consider the legality of strikes, the language used by government and military officials, the target selection procedures, intelligence collection methods and finally the different styles of strikes used to target individuals. By analysing these main, major components of the campaign, this study will conclude that, throughout Barack Obama’s presidency, the campaign was not as not as effective as it should have been. By revising and updating many of these factors analysed, US operations in the Middle East could become more effective in eliminating al Qaeda and associated forces

Uniqueness of Ground States for Short-Range Spin Glasses in the Half-Plane

We consider the Edwards-Anderson Ising spin glass model on the half-plane $Z \times Z^+$ with zero external field and a wide range of choices, including mean zero Gaussian, for the common distribution of the collection J of i.i.d. nearest neighbor couplings. The infinite-volume joint distribution $K(J,\alpha)$ of couplings J and ground state pairs $\alpha$ with periodic (respectively, free) boundary conditions in the horizontal (respectively, vertical) coordinate is shown to exist without need for subsequence limits. Our main result is that for almost every J, the conditional distribution $K(\alpha|J)$ is supported on a single ground state pair.Comment: 20 pages, 3 figure

Marking (1,2) Points of the Brownian Web and Applications

The Brownian web (BW), which developed from the work of Arratia and then T\'{o}th and Werner, is a random collection of paths (with specified starting points) in one plus one dimensional space-time that arises as the scaling limit of the discrete web (DW) of coalescing simple random walks. Two recently introduced extensions of the BW, the Brownian net (BN) constructed by Sun and Swart, and the dynamical Brownian web (DyBW) proposed by Howitt and Warren, are (or should be) scaling limits of corresponding discrete extensions of the DW -- the discrete net (DN) and the dynamical discrete web (DyDW). These discrete extensions have a natural geometric structure in which the underlying Bernoulli left or right "arrow" structure of the DW is extended by means of branching (i.e., allowing left and right simultaneously) to construct the DN or by means of switching (i.e., from left to right and vice-versa) to construct the DyDW. In this paper we show that there is a similar structure in the continuum where arrow direction is replaced by the left or right parity of the (1,2) space-time points of the BW (points with one incoming path from the past and two outgoing paths to the future, only one of which is a continuation of the incoming path). We then provide a complete construction of the DyBW and an alternate construction of the BN to that of Sun and Swart by proving that the switching or branching can be implemented by a Poissonian marking of the (1,2) points.Comment: added 3 references to Sections 1, 2, 3; expanded explanations in Subsections 7.3, 7.4, 7.