Distributed Computing on Core-Periphery Networks: Axiom-based Design

Inspired by social networks and complex systems, we propose a core-periphery network architecture that supports fast computation for many distributed algorithms and is robust and efficient in number of links. Rather than providing a concrete network model, we take an axiom-based design approach. We provide three intuitive (and independent) algorithmic axioms and prove that any network that satisfies all axioms enjoys an efficient algorithm for a range of tasks (e.g., MST, sparse matrix multiplication, etc.). We also show the minimality of our axiom set: for networks that satisfy any subset of the axioms, the same efficiency cannot be guaranteed for any deterministic algorithm

Multiplicative Attribute Graph Model of Real-World Networks

Large scale real-world network data such as social and information networks are ubiquitous. The study of such social and information networks seeks to find patterns and explain their emergence through tractable models. In most networks, and especially in social networks, nodes have a rich set of attributes (e.g., age, gender) associated with them. Here we present a model that we refer to as the Multiplicative Attribute Graphs (MAG), which naturally captures the interactions between the network structure and the node attributes. We consider a model where each node has a vector of categorical latent attributes associated with it. The probability of an edge between a pair of nodes then depends on the product of individual attribute-attribute affinities. The model yields itself to mathematical analysis and we derive thresholds for the connectivity and the emergence of the giant connected component, and show that the model gives rise to networks with a constant diameter. We analyze the degree distribution to show that MAG model can produce networks with either log-normal or power-law degree distributions depending on certain conditions.Comment: 33 pages, 6 figure

The organization of the interbank network and how ECB unconventional measures affected the e-MID overnight market

The topological properties of interbank networks have been discussed widely in the literature mainly because of their relevance for systemic risk. Here we propose to use the Stochastic Block Model to investigate and perform a model selection among several possible two block organizations of the network: these include bipartite, core-periphery, and modular structures. We apply our method to the e-MID interbank market in the period 2010-2014 and we show that in normal conditions the most likely network organization is a bipartite structure. In exceptional conditions, such as after LTRO, one of the most important unconventional measures by ECB at the beginning of 2012, the most likely structure becomes a random one and only in 2014 the e-MID market went back to a normal bipartite organization. By investigating the strategy of individual banks, we explore possible explanations and we show that the disappearance of many lending banks and the strategy switch of a very small set of banks from borrower to lender is likely at the origin of this structural change.Comment: 33 pages, 5 figure

Long-Term Results after DMEK (Descemet’s Membrane Endothelial Keratoplasty)

Ziel der Arbeit: Evaluation der langfristigen Ergebnisse sowie der Komplikationsrate nach Descemet’s Membran Endothelialen Keratoplastik (DMEK) Methoden: Eine cross-sectional, Fall-Serien Studie. Insgesamt wurden 230 Augen von 142 Patienten, die zwischen 2010 und 2014 eine DMEK an der Universitäts-Augenklinik Marburg bekommen haben, untersucht. Die best-korrigierte Sehschärfe (BCVA), die Refraktion, die zentrale Hornhautdicke, das Hornhautvolumen sowie die Endothelialzelldichte wurden als Parameter herangezogen und mit den präoperativen Befunden verglichen. Die Transplantat-Überlebensrate sowie die postoperativen Komplikationen wurden ebenfalls betrachtet. Ergebnisse: Die Nachbeobachtungszeit betrug 47 ± 13.3 Monate. Bei den Patienten die keine anderen okuläre Erkrankungen hatten hat sich die BCVA von 0.60 ± 0.32 logMAR präoperativ auf bis zu 0.10 ± 0.22 logMAR verbessert (201 Augen). 71.1% dieser Patienten hatten eine BCVA von 0.11 logMAR oder besser (≥ 0.8 dezimal), wobei 49.2% dieser Patienten eine volle BCVA von 0.00 logMAR oder besser erreicht haben. Die zentrale Hornhautdicke hat von 675 ± 112µm präoperativ auf 547 ± 52 µm in der letzten Follow-up Untersuchung abgenommen, und das Hornhautvolumen hat von 65.2 ± 8.4 mm2 präoperativ auf 61.9 ± 5.4 mm2 abgenommen. Der Endothelzellverlust lag bei 1392 ± 455 Zellen/mm², was einem durchschnittlichen Verlust von 54.7% der Transplantatzellen entspricht. Die Transplantat-Überlebensrate lag bei 92% mit einer durchschnittlichen Überlebenszeit von 76.6 ± 1.3 Monaten. Schlussfolgerung: DMEK bietet hohe visuelle Ergebnisse und sehr gute klinische Befunde, die mehrere Jahre nach der Operation stabil bleiben können. Durch die hohe Transplantat-Überlebensrate und die niedrige postoperative Komplikationsrate wird DMEK derzeit als erste Wahl zur Behandlung von Endothelzellerkrankungen eingesetzt

Exploring the Impact of Socio-Technical Core-Periphery Structures in Open Source Software Development

In this paper we apply the social network concept of core-periphery structure to the sociotechnical structure of a software development team. We propose a socio-technical pattern that can be used to locate emerging coordination problems in Open Source projects. With the help of our tool and method called TESNA, we demonstrate a method to monitor the socio-technical core-periphery movement in Open Source projects. We then study the impact of different core-periphery movements on Open Source projects. We conclude that a steady core-periphery shift towards the core is beneficial to the project, whereas shifts away from the core are clearly not good. Furthermore, oscillatory shifts towards and away from the core can be considered as an indication of the instability of the project. Such an analysis can provide developers with a good insight into the health of an Open Source project. Researchers can gain from the pattern theory, and from the method we use to study the core-periphery movements