Sketch-based Influence Maximization and Computation: Scaling up with Guarantees

Propagation of contagion through networks is a fundamental process. It is used to model the spread of information, influence, or a viral infection. Diffusion patterns can be specified by a probabilistic model, such as Independent Cascade (IC), or captured by a set of representative traces. Basic computational problems in the study of diffusion are influence queries (determining the potency of a specified seed set of nodes) and Influence Maximization (identifying the most influential seed set of a given size). Answering each influence query involves many edge traversals, and does not scale when there are many queries on very large graphs. The gold standard for Influence Maximization is the greedy algorithm, which iteratively adds to the seed set a node maximizing the marginal gain in influence. Greedy has a guaranteed approximation ratio of at least (1-1/e) and actually produces a sequence of nodes, with each prefix having approximation guarantee with respect to the same-size optimum. Since Greedy does not scale well beyond a few million edges, for larger inputs one must currently use either heuristics or alternative algorithms designed for a pre-specified small seed set size. We develop a novel sketch-based design for influence computation. Our greedy Sketch-based Influence Maximization (SKIM) algorithm scales to graphs with billions of edges, with one to two orders of magnitude speedup over the best greedy methods. It still has a guaranteed approximation ratio, and in practice its quality nearly matches that of exact greedy. We also present influence oracles, which use linear-time preprocessing to generate a small sketch for each node, allowing the influence of any seed set to be quickly answered from the sketches of its nodes.Comment: 10 pages, 5 figures. Appeared at the 23rd Conference on Information and Knowledge Management (CIKM 2014) in Shanghai, Chin

Factors affecting metal mobilisation during oxidation of sulphidic, sandy wetland substrates

Most metals accumulate as sulphides under anoxic conditions in wetland substrates, reducing their bioavailability due to the solubility of metal sulphides. However, upon oxidation of these sulphides when the substrate is occasionally oxidised, metals can be released from the solid phase to the pore water or overlaying surface water. This release can be affected by the presence of carbonates, organic matter and clay. We compared changes of Cd, Cu and Zn mobility (CaCl2 extraction) during oxidation of a carbonate-rich and a carbonate-poor sulphidic, sandy wetland substrate. In addition, we studied how clay with low and high cation sorption capacity (bentonite and kaolinite, respectively) and organic matter (peat) can counteract Cd, Cu and Zn release during oxidation of both carbonate-rich and carbonate-poor sulphidic sediments. CaCl2-extractability of Cu, a measure for its availability, is low in both carbonate-poor and carbonate-rich substrates, whereas its variability is high. The availability of Cd and Zn is much higher and increases when peat is supplied to carbonate-poor substrates. A strong reduction of Cd and Zn extractability is observed when clay is added to carbonate-poor substrates. This reduction depends on the clay type. Most observations could be explained taking into account pH differences between treatments, with kaolinite resulting in a lower pH in comparison to bentonite. These pH differences affect the presence and characteristics of dissolved organic carbon and the metal speciation, which in turns affects the interaction of metals with the solid soil phase. In carbonate-rich substrates, Cd and Zn availability is lower and the effects of peat and clay amendment are less clear. The latter can also be attributed to the high pH and lack of pH differences between treatments

Wetenskap op weg na 'n nuwe identiteit en samewerking

A traditional approach in Calvinist philosophy is to start with an analysis of cosmic diversity in order to find the identity of a particular phenomenon, and then to end with an account of the inter-relatedness between the phenomenon and other things. Although Stoker's philosophy tries to keep a balance between diversity and coherence as an entrée to reality - his later publications reflected a remarkable shift hy emphasising the coherence and contextual meaning of things. Taking the coherence and contextual meaning of things as the point of departure, this article explores a converse version of the traditional approach by using coherence (and not identity) as a means to sound out some new meanings of science and scientific co-operation. An indication of transdisciplinary co-operation is rendered in terms of the preponderance of reality‘s call upon man to be known and cared for. The article ends with a few examples of unusual concepts that may serve as a revised index to identify world-friendly and humane scientific activities

Grepe uit die eksistensialistiese etiek

In die jongste tyd is die moderne mens doodbang vir alle etiese programme, vir ’n Etiek waarin vir alle mense en tye voorgeskrewe reels, hoe ’n mens in elke situasie moet handel, gegee word

Toekomskunde as morele verantwoordelikheid

Vir meeste van ons kortsigtiges het die jaar 2000 ’n veraf, ma- giese en newelagtige assosiasie. Ons praat baie makliker van ons er- varing van en herinnering aan die Smutsregering en die Tweede Wêreldoorlog wat 32 jaar gelede geëindig het. Maar ons voer vol- struispolitiek oor die jaar 2000 wat maar 23 jaar om die draai lê. Oor 23 jaar is die huidige geslag studente so oud soos baie van ons huidige personeel.

Die verkryging van getuienis deur middel van gedwonge chirurgie

This contribution considers the question of whether South African law permits the compelled surgical intrusion into a suspect’s body in order to obtain evidence. The courts came to contradictory findings regarding this issue. Such a procedure undoubtedly constitutes an infringement of a suspect’s right to privacy and the right to freedom and security of the person. It can only be tolerated if the requirements of the constitutional limitations clause are complied with. The authors support the approach that forced surgical interventions in order to obtain evidence are not permitted by the Criminal Procedure Act and that the legislature should intervene to provide for it. Recommendations as to the approach that could be followed are made