Orthogonal nets and Clifford algebras

A Clifford algebra model for M"obius geometry is presented. The notion of Ribaucour pairs of orthogonal systems in arbitrary dimensions is introduced, and the structure equations for adapted frames are derived. These equations are discretized and the geometry of the occuring discrete nets and sphere congruences is discussed in a conformal setting. This way, the notions of ``discrete Ribaucour congruences'' and ``discrete Ribaucour pairs of orthogonal systems'' are obtained --- the latter as a generalization of discrete orthogonal systems in Euclidean space. The relation of a Cauchy problem for discrete orthogonal nets and a permutability theorem for the Ribaucour transformation of smooth orthogonal systems is discussed.Comment: Plain TeX, 16 pages, 4 picture

Cell Stress during Development and Disease - Special Issue, Review article Balancing DNA repair to prevent ageing and cancer

DNA damage is a constant stressor to the cell. Persistent damage to the DNA over time results in an increased risk of mutation and an accumulation of mutations with age. Loss of efficient DNA damage repair can lead to accelerated ageing phenotypes or an increased cancer risk, and the trade-off between cancer susceptibility and longevity is often driven by the cell's response to DNA damage. High levels of mutations in DNA repair mutants often leads to excessive cell death and stem cell exhaustion which may promote premature ageing. Stem cells themselves have distinct characteristics that enable them to retain low mutation rates. However, when mutations do arise, stem cell clonal expansion can also contribute to age-related tissue dysfunction as well as heightened cancer risk. In this review, we will highlight increasing DNA damage and mutation accumulation as hallmarks common to both ageing and cancer. We will propose that anti-ageing interventions might be cancer preventative and discuss the mechanisms through which they may act

Accumulation of heavy metals from single and mixed metal solutions by the gastropod mollusc Tympanotonus fuscatus linnaeus from a Niger Delta estuary: Implications for biomonitoring

The usefulness of the intertidal gastropod Tympanotonus fuscatus L as a biomonitor of heavy metals in tropical estuaries was assessed. The periwinkles were collected from a site in the upper Bonny Estuary, Southern Nigeria and exposed in a series of experiments either singly or binary mixtures to copper, zinc and cadmium. The accumulation of Cd was monotonic with increase in exposure concentration with a highly significant linear regression (r2 = 0.999, pZn>Cd but those for Cu and Zn showed reduction as the exposure concentration increased suggesting regulation of these metals. In accumulation from binary mixtures, Cu was synergistic to Cdaccumulation in combinations involving 0.05 mg/L Cu but antagonistic for 0.5 and 1.0 mg/L Cu combinations. Zn was antagonistic to Cd accumulation in mixtures with 0.05 and 1.0 mg/L Zn. Cd didnot show any effect on Zn accumulation in any of the mixtures but it was antagonistic to Cu accumulation in some combinations (1.0 mg/L Cu + 0.05, 1.0 mg/L Cd). It is concluded that T. fuscatusis a good candidate for the biomonitoring of Cd but not for Zn and Cu. The ambient concentrations of Cu and Zn may affect the accumulation of Cd, and need to be considered in the interpretation of Cddata in T. fuscatus

A Sparse Stress Model

Force-directed layout methods constitute the most common approach to draw general graphs. Among them, stress minimization produces layouts of comparatively high quality but also imposes comparatively high computational demands. We propose a speed-up method based on the aggregation of terms in the objective function. It is akin to aggregate repulsion from far-away nodes during spring embedding but transfers the idea from the layout space into a preprocessing phase. An initial experimental study informs a method to select representatives, and subsequent more extensive experiments indicate that our method yields better approximations of minimum-stress layouts in less time than related methods.Comment: Appears in the Proceedings of the 24th International Symposium on Graph Drawing and Network Visualization (GD 2016

High-Throughput Computational Screening of Cubic Perovskites for Solid Oxide Fuel Cell Cathodes

It is a present-day challenge to design and develop oxygen-permeable solid oxide fuel cell (SOFC) electrode and electrolyte materials that operate at low temperatures. Herein, by performing high-throughput density functional theory calculations, oxygen vacancy formation energy, Evac, data for a pool of all-inorganic ABO3 and A I 0.5 A II 0.5 BO3 cubic perovskites is generated. Using E vac data of perovskites, the area-specific resistance (ASR) data, which is related to both oxygen reduction reaction activity and selective oxygen ion conductivity of materials, is calculated. Screening a total of 270 chemical compositions, 31 perovskites are identified as candidates with properties that are between those of state-of-the-art SOFC cathode and oxygen permeation components. In addition, an intuitive approach to estimate Evac and ASR data of complex perovskites by using solely the easy-to-access data of simple perovskites is shown, which is expected to boost future explorations in the perovskite material search space for genuinely diverse energy applications.</p

Ensuring water supply for all towns and villages in the Eastern Cape and Western Cape Provinces of South Africa

The Department of Water Affairs (DWA) has embarked on a nationwide programme to develop water-reconciliation strategies for all towns across the country. Reconciliation strategies for the major metropolitan areas and systems (e.g. Johannesburg/Pretoria, East London, Cape Town and Durban) were developed next. The implementation of these strategies is monitored by strategy steering committees. The approach has now been extended to all other metropolitan areas as well as all towns and villages or clusters of villages. In order to prepare the actual strategies, regardless of the size of the town, thorough documentation, research and analysis of the available information was required, as well as evaluation of projected growth scenarios to assess water requirements over the next 25 years and identification of potential additional sources to meet this growing requirement. It has emerged that the poor operation and maintenance of water supply, treatment and reticulation infrastructure are resulting in significant losses, which, if corrected, can reverse the current water shortages being experienced. Similarly, the generally poor management of effluent remains a threat to surface water and groundwater quality downstream. It appears that many municipalities perceive groundwater as an unreliable resource; however, in general, the issue of staff and skills shortages to manage the resource effectively is the actual problem. This is an operational issue rather than a groundwater-resource-specific issue. This aspect requires special attention for existing groundwater schemes and proposed groundwater development. In most instances water conservation and water-demand management and the development of local surface and groundwater resources are the most feasible options to meet any current or projected future water-supply shortfalls. Any intervention must be combined with a skills-development programme at the operational level to ensure the sustainability of the proposed supply options. This paper is based on the experience gained in the development of reconciliation strategies for the towns and villages in the DWA Southern Planning Region (i.e. surface water drainage areas in the Eastern Cape and Western Cape Provinces), which was carried out by Umvoto Africa in association with engineering consulting practice Aurecon