Random Topologies and the emergence of cooperation: the role of short-cuts

We study in detail the role of short-cuts in promoting the emergence of cooperation in a network of agents playing the Prisoner's Dilemma Game (PDG). We introduce a model whose topology interpolates between the one-dimensional euclidean lattice (a ring) and the complete graph by changing the value of one parameter (the probability p to add a link between two nodes not already connected in the euclidean configuration). We show that there is a region of values of p in which cooperation is largely enhanced, whilst for smaller values of p only a few cooperators are present in the final state, and for p \rightarrow 1- cooperation is totally suppressed. We present analytical arguments that provide a very plausible interpretation of the simulation results, thus unveiling the mechanism by which short-cuts contribute to promote (or suppress) cooperation

The Nullity of Bicyclic Signed Graphs

Let \Gamma be a signed graph and let A(\Gamma) be the adjacency matrix of \Gamma. The nullity of \Gamma is the multiplicity of eigenvalue zero in the spectrum of A(\Gamma). In this paper we characterize the signed graphs of order n with nullity n-2 or n-3, and introduce a graph transformation which preserves the nullity. As an application we determine the unbalanced bicyclic signed graphs of order n with nullity n-3 or n-4, and signed bicyclic signed graphs (including simple bicyclic graphs) of order n with nullity n-5

Advances In Ti6Al4V Additive Manufacturing in South Africa

Published Conference ProceedingsSouth Africa has the potential to add significant value to its titanium natural resource. Initiated by a National Research and Technology Foresight Project in 1999, a national consensus has grown regarding this beneficiation opportunity. It culminated in the establishment of the Titanium Centre of Competence in 2009. Additive manufacturing was included as technology platform of the Titanium Centre of Competence. In 2013 the Department of Science and Technology commissioned the development of a South African Additive Manufacturing Technology Roadmap. This paper elaborates on the growth of additive manufacturing in South Africa, emphasising the successes achieved with producing customised medical implants from Ti6Al4V powder. Examples of internationally leading work on such implants in maxillofacial reconstructive surgery are given. The national Collaborative Programme in Additive Manufacturing and the sub-programme on qualification of Ti6Al4V medical implants and aerospace parts produced through additive manufacturing, are discussed. Initial achievements of this programme are shared

Detection of OD towards the low-mass protostar IRAS16293-2422

Although water is an essential and widespread molecule in star-forming regions, its chemical formation pathways are still not very well constrained. Observing the level of deuterium fractionation of OH, a radical involved in the water chemical network, is a promising way to infer its chemical origin. We aim at understanding the formation mechanisms of water by investigating the origin of its deuterium fractionation. This can be achieved by observing the abundance of OD towards the low-mass protostar IRAS16293-2422, where the HDO distribution is already known. Using the GREAT receiver on board SOFIA, we observed the ground-state OD transition at 1391.5 GHz towards the low-mass protostar IRAS16293-2422. We also present the detection of the HDO 111-000 line using the APEX telescope. We compare the OD/HDO abundance ratio inferred from these observations with the predictions of chemical models. The OD line is detected in absorption towards the source continuum. This is the first detection of OD outside the solar system. The SOFIA observation, coupled to the observation of the HDO 111-000 line, provides an estimate of the abundance ratio OD/HDO ~ 17-90 in the gas where the absorption takes place. This value is fairly high compared with model predictions. This may be reconciled if reprocessing in the gas by means of the dissociative recombination of H2DO+ further fractionates OH with respect to water. The present observation demonstrates the capability of the SOFIA/GREAT instrument to detect the ground transition of OD towards star-forming regions in a frequency range that was not accessible before. Dissociative recombination of H2DO+ may play an important role in setting a high OD abundance. Measuring the branching ratios of this reaction in the laboratory will be of great value for chemical models.Comment: 6 pages, 6 figures, 3 tables, accepted for publication in A&A SOFIA/GREAT special issu

Effective Mass of the Four Flux Composite Fermion at ν=1/4\nu = 1/4

We have measured the effective mass ($m^*$) of the four flux composite fermion at Landau level filling factor $\nu = 1/4$ ($^4$CF), using the activation energy gaps at the fractional quantum Hall effect (FQHE) states $\nu$ = 2/7, 3/11, and 4/15 and the temperature dependence of the Shubnikov-de Haas (SdH) oscillations around $\nu = 1/4$. We find that the energy gaps show a linear dependence on the effective magnetic field $B_{eff}$ ($\equiv B-B_{\nu=1/4}$), and from this linear dependence we obtain $m^* = 1.0 m_e$ and a disorder broadening $\Gamma \sim$ 1 K for a sample of density $n = 0.87 \times 10^{11}$ /cm$^2$. The $m^*$ deduced from the temperature dependence of the SdH effect shows large differences for $\nu > 1/4$ and $\nu < 1/4$. For $\nu > 1/4$, $m^* \sim 1.0 m_e$. It scales as $\sqrt{B_{\nu}}$ with the mass derived from the data around $\nu =1/2$ and shows an increase in $m^*$ as $\nu \to 1/4$, resembling the findings around $\nu =1/2$. For $\nu < 1/4$, $m^*$ increases rapidly with increasing $B_{eff}$ and can be described by $m^*/m_e = -3.3 + 5.7 \times B_{eff}$. This anomalous dependence on $B_{eff}$ is precursory to the formation of the insulating phase at still lower filling.Comment: 5 pages, 3 figure

A comparison of soil moisture relations between standing and clearfelled plots with burnt and unburnt harvest residue treatments of a clonal eucalypt plantation on the Zululand Coastal Plain, South Africa

The effects of clearfelling and subsequent residue retention or burning on water and nutrient balances needs to be understood and quantified on forest sites that are sensitive to loss, so that the long-term sustainable productivity of such sites can be maintained and promoted. An&nbsp; experimental site was established in a clonal eucalypt compartment on the Zululand Coastal Plain, to compare changes in water fluxes through the mature undisturbed eucalypt stand with those after felling and re-planting, under 2 conditions: burning, and retention of the harvesting residues. The study was located in an area of high rainfall and high stand productivity, with sandy soils and low soil carbon and nutrient status; chosen so that the effects of intensive demands on water and nutrient fluxes on a potentially sensitive site could be investigated. This paper presents only the hydrological component of the study. Data collection included weekly determination of rainfall, throughfall, stemflow and soil moisture fluxes from the surface to a depth of 1 m. Drainage rates through the profile were established using time domain reflectometry probes while water drainage volumes were assessed using shallow plate lysimeters. Despite slow growth in the unfelled crop during the monitoring period (attributed to a pest infestation), soil moisture depletion remained rapid and drainage below 1 m remained low. Soil moisture was recharged within a few months after clearfelling, but became rapidly depleted as the canopy of new crop developed and approached canopy closure. A decreased wetting-front velocity and a marginally higher field capacity were proposed as evidence of pore clogging that appeared to occur during the inter-rotation period. The soil profile under the unburnt residue maintained a marginally higher soil moisture status and lower drainage than the soil profile under the burnt residue. Although soil moisture and drainage in the burnt and unburnt residue treatments became similar to the unfelled crop from canopy closure onwards, rainfall additions to soil moisture were depleted faster under the new crop during the first few months after canopy closure. Small differences in soil moisture status between the burnt and unburnt residue treatments presented here may not be sufficient to influence residue management decisions. The length of the inter-rotation period and practice of residue burning may, however, need consideration where soil carbon and nutrient loss or displacement may negatively affect the sustainability of the site.Keywords: Soil moisture drainage, residue burning, post felling, inter-rotatio

Characterisation of Ti6Al4V (ELI) Powder Used by the South African Collaborative Program in Additive

Conference ProceedingsThe South African Additive Manufacturing Strategy recommends research and development towards “Qualified AM parts for medical and aerospace” as one of the national priorities. In response to this, the national Collaborative Program in Additive Manufacturing includes a sub-programme on “Qualification of Additive Manufacturing of Ti6Al4V for Medical Implants and Aerospace Parts”. This sub-programme entails comprehensive systematic research aimed at establishing a database of material and process data needed for qualification of the AM powders, AM processes and post-process treatments used for medical and aerospace applications. The paper describes the approach taken to characterise the Ti6Al4V (ELI) powder in the as received state and after exposure during repeated AM build cycles, by determining the physical and chemical properties of powder used in two selective laser melting (SLM) machines and a laser engineered net shaping (LENS) machine. Properties of the powder that were determined include the particle size, particle size distribution, morphology, oxygen and nitrogen gas content, and elemental composition. Characterisation techniques employed are Scanning Electron Microscopy, Laser Diffraction, Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES), and Inert Gas Fusion. The results of the analyses on as-received powder and powder after exposure to numerous build cycles in the different AM machines are presented and discussed