Vertex and edge covers with clustering properties: complexity and algorithms

We consider the concepts of a t-total vertex cover and a t-total edge cover (t≥1), which generalise the notions of a vertex cover and an edge cover, respectively. A t-total vertex (respectively edge) cover of a connected graph G is a vertex (edge) cover S of G such that each connected component of the subgraph of G induced by S has at least t vertices (edges). These definitions are motivated by combining the concepts of clustering and covering in graphs. Moreover they yield a spectrum of parameters that essentially range from a vertex cover to a connected vertex cover (in the vertex case) and from an edge cover to a spanning tree (in the edge case). For various values of t, we present NP-completeness and approximability results (both upper and lower bounds) and FTP algorithms for problems concerned with finding the minimum size of a t-total vertex cover, t-total edge cover and connected vertex cover, in particular improving on a previous FTP algorithm for the latter problem

Birch in Swedish Forestry : a study of the production potential of birch timber

Björk är Sveriges vanligaste lövträd och utgör cirka 13 % av landets stående virkesförråd. Intresset för trädslaget har varierat historiskt men har med tiden ökat på grund av att till exempel certifieringskrav, klimatförändringar och skadegörare fordrar en större variation i trädslagsblandning. Detta för att kunna bedriva ett hållbart skogsbruk samt öka skogens biologiska mångfald. Trots björkråvarans potential att bli sågtimmer omvandlas den främst till massaved och ytterst små volymer sågas till timmer. Denna intervjustudie syftar till att studera timmerproduktion av björk i Sverige och bidra till en djupare förståelse över björktimrets användning idag samt hur marknaden för björkvirke kan utvecklas. Studien baseras på intervjuer med virkesköpare från olika företag och lövsågverk utspridda geografiskt över Sverige. Studien visar att björkråvara av bra kvalitet påträffas i liten utsträckning i skogen enligt virkesköparna och återfinns generellt i relativt så volymer insprängt med andra barrträd. I Sverige är det vanligast med naturligt föryngrad björk som har en sämre kvalitet än planterad förädlad björk. Det råder delade meningar om det finns en efterfrågan på björktimmer men i intervjuerna uttrycker de större sågverken att de upplever en brist på björkråvara. Både virkesköpare och lövsågverk indikerar att björkråvara lämplig för timmerproduktion går förlorad till massaindustrin istället för att förädlas. Trots att sågverken har höjt björktimmerpriset har de svårt att konkurrera med dagens höga massavedpriser. En stor del av problematiken för björktimmerindustrin rör en bristande lönsamheten för uttag av de små volymer kvalitativ björk som återfinns vid avverkning samt att antalet lövsågverk är få i Sverige. Detta bidrar till långa och dyra transporter i de flesta fall. Sågverken uttryckte en positiv framtidssyn för björktimmer och att intresset för björk ökar. Enligt virkesköpare är flera markägare intresserade av att plantera förädlad björk som ett alternativt trädslag till barrträden.Birch is Sweden's most common deciduous tree species and constitutes about 13 % of the country's forest stand volume. The interest in this species has varied throughout history, however it has increased the last few years due to certification requirements, climate change and pests which demand a greater variety of tree species in forest stands to ensure sustainable forestry practices and increase biodiversity. Despite the potential of birch raw material, it is primarily converted to pulpwood and very small volumes are grown in larger dimensions for timber production. This interview-based study aims to examine the production of birch timber in Sweden and will contribute to a deeper understanding of the current use of birch timber and how the market for birch timber can be developed. The study consists of interviews with timber buyers from different companies and hardwood sawmills that are spread geographically across Sweden. The study shows that good quality birch, suitable for timber production is found in limited extent in the forest according to timber buyers and is generally found in relatively small volumes interspersed with coniferous trees. In Sweden, naturally regenerated birch is more common and has lower quality than planted and improved birch material. There are differing opinions about whether there is a demand for birch timber, but the bigger sawmills expressed in the interviews that they experience a shortage of birch sawlogs. Both timber buyers and hardwood sawmills indicate that birch trees suitable for timber production is lost to the pulp industry instead of being sawn into products with higher value. Despite the sawmills raising the price of birch timber, they struggle to compete with the high price of pulpwood. Much of the problem for the birch timber industry is the lack of profitability in extracting the small volumes of quality birch found in harvested areas and the few numbers of hardwood sawmills that exist in Sweden, which adds up to complicated and expensive transportations in most cases. When asked of the future of birch timber the sawmills expressed a positive outlook for the future and predicted an increase in interest in birch timber. According to timber buyers, several landowners are interested in planting genetically improved birch as an alternative tree species to coniferous trees

Corticothalamic projections control synchronization in locally coupled bistable thalamic oscillators

Thalamic circuits are able to generate state-dependent oscillations of different frequencies and degrees of synchronization. However, only little is known how synchronous oscillations, like spindle oscillations in the thalamus, are organized in the intact brain. Experimental findings suggest that the simultaneous occurrence of spindle oscillations over widespread territories of the thalamus is due to the corticothalamic projections, as the synchrony is lost in the decorticated thalamus. Here we study the influence of corticothalamic projections on the synchrony in a thalamic network, and uncover the underlying control mechanism, leading to a control method which is applicable in wide range of stochastic driven excitable units.Comment: 4 pages with 4 figures (Color online on p.3-4) include

Memory for nonadjacent dependencies in the first year of life and its relation to sleep

Grammar learning requires memory for dependencies between nonadjacent elements in speech. Immediate learning of nonadjacent dependencies has been observed in very young infants, but their memory of such dependencies has remained unexplored. Here we used event-related potentials to investigate whether 6- to 8-month-olds retain nonadjacent dependencies and if sleep after learning affects this memory. Infants were familiarised with two rule-based morphosyntactic dependencies, presented in sentences of an unknown language. Brain responses after a retention period reveal memory of the nonadjacent dependencies, independent of whether infants napped or stayed awake. Napping, however, altered a specific processing stage, suggesting that memory evolves during sleep. Infants with high left frontal spindle activity show an additional brain response indicating memory of individual speech phrases. Results imply that infants as young as 6 months are equipped with memory mechanisms relevant to grammar learning. They also suggest that during sleep, consolidation of highly specific information can co-occur with changes in the nature of generalised memory

Bifrontal transcranial direct current stimulation slows reaction time in a working memory task

BACKGROUND: Weak transcortical direct current stimulation (tDCS) applied to the cortex can shift the membrane potential of superficial neurons thereby modulating cortical excitability and activity. Here we test the possibility of modifying ongoing activity associated with working memory by tDCS. The concept of working memory applies to a system that is capable of transiently storing and manipulating information, as an integral part of the human memory system. We applied anodal and cathodal transcranial direct current (tDCS) stimulation (260 μA) bilaterally at fronto-cortical electrode sites on the scalp over 15 min repeatedly (15 sec-on/15 sec-off) as well as sham-tDCS while subjects performed a modified Sternberg task. RESULTS: Reaction time linearly increased with increasing set size. The slope of this increase was closely comparable for real and sham stimulation indicating that our real stimulation did not effect time required for memory scanning. However, reaction time was slowed during both anodal and cathodal stimulation as compared to placebo (p < 0.05) indicating that real stimulation hampered neuronal processing related to response selection and preparation. CONCLUSION: Intermittent tDCS over lateral prefrontal cortex during a working memory task impairs central nervous processing related to response selection and preparation. We conclude that this decrease in performance by our protocol of intermittent stimulation results from an interference mainly with the temporal dynamics of cortical processing as indexed by event-related sustained and oscillatory EEG activity such as theta

Zur Bedeutung von Amphibienlarven für die Populationsentwicklung des Gemeinen Gelbrandkäfers <em>Dytiscus marginalis</em>, L. 1758

Die vorliegende Arbeit beschäftigt sich mit der Frage, inwiefern Amphibienlarven sich auf die Populationsentwicklung und Verbreitung des Gemeinen Gelbrandkäfers Dytiscus marginalis auswirken können. Dazu wurden in drei getrennten Untersuchungsgebieten und im Labor zahlreiche Indizien für eine eventuelle Abhängigkeit des Käfers von diesen Beutetieren überprüft. Im Freiland konnten u.a. die Verbreitungsmuster und Fluktuationen beider Tiergruppen präzise dokumentiert werden. Dies geschah ohne Unterbrechung von 1992 bis 2000 in 7 Gewässern einer südlich von Bonn gelegen Agrarlandschaft und von 1995 bis 1997 in 9 Gewässern in einem westlich von Bonn gelegenen Staatsforst. Dafür wurde eine besonders struktur- und tierschonende Lebendfangmethode, die sog. Auftauchfalle, konstruiert. Für die statistische Abschätzung der Käferpopulationen wurde eine der Biologie des D.°marginalis und den vorliegenden Erfassungsmöglichkeiten angepaßte robuste Fang-Markierungs-Wiederfang-Methode für langfristige Schätzungen mit kleinen Stichproben entwickelt. Zur Überprüfung der in den ersten beiden Untersuchungsgebieten beobachteten Verteilungsmuster wurden 1998 noch zusätzlich die betreffenden Tierbestände in 26 Kleingewässern dreier etwa 50 km entfernt gelegener Mittelgebirgstäler erfaßt. Zwecks vergleichender Untersuchungen unter definierten Bedingungen wurde eine Batterie von 24 gleichartigen Versuchsaquarien aufgestellt. Dort wurden die Entwicklungserfolge kleiner D.°marginalis-Larven-Populationen bei unterschiedlichem Nahrungsangebot ermittelt. Außerdem wurde in diesen Aquarien nach Unterschieden bzw. Gemeinsamkeiten der Phänologien verschiedener Dytiscus-Arten im Vergleich zu heimischen Anuren gesucht. Die Freilanderhebungen lieferten gleiche Verbreitungsmuster des Käfers mit der Summe der frühzeitig im Jahr ablaichenden Froschlurcharten Bufo bufo, Rana temporaria und Rana dalmatina in allen drei Untersuchungsgebieten. Besonders in den künstlich angelegten Kleinteichen der Agrarlandschaft konnte präzise die Synchronverteilung des Räubers mit den Kaulquappen dieser drei Beutetierarten sowie ihr gemeinsames Verschwinden im Verlauf der Untersuchungen dokumentiert werden. Die Laboruntersuchungen zeigten, daß die gemeinsamen Verbreitungsmuster dieser Tiergruppen auf eine direkte Abhängigkeit der Käferlarven von den Kaulquappen der betreffenden Arten zurückzuführen sind. So hing in den Versuchsaquarien der Entwicklungserfolg einer Dytiscus-Larven-Population stets ausschließlich von der Dichte der Amphibienlarven ab und wurde durch den Kannibalismus der Käferlarven selbstreguliert. Weiterhin konnte sowohl im Freiland als auch im Labor eine präzise Adaptierung der Entwicklungszeiten des D.°marginalis an jene der frühzeitig ablaichenden Froschlurche beobachtet und als direkte Anpassung des Käfers beschrieben werden. Außerdem wurde die im Vergleich mit interspezifischen Nahrungskonkurrenten ausgesprochen gute und nachhaltige Ressourcennutzung des Gelbrandkäfers dokumentiert. Diese und alle weiteren hier untersuchten Indizien sprechen eindeutig dafür, daß der D.°marginalis zwar ein breites Nahrungsspektrum nutzen kann, für seine Larvalentwicklung aber unbedingt auf Amphibienlarven angewiesen ist. Der große Erfolg dieser Käferart ist darin begründet, daß sie sich im Gegensatz zu anderen Arten ihrer Gattung auf die ebenfalls besonders häufigen und weit verbreiteten „Frühlaicher“ unter den Froschlurchen spezialisiert hat. Dafür wurden weitere Anpassungsstrategien entwickelt, von denen einige hier aufgedeckt werden konnten.The importance of amphibian larvae in the population development of the large diving beetle Dytiscus marginalis, L. 1758 The present study addresses the extent to which amphibian larvae can affect the population development and distribution of the large diving beetle Dytiscus marginalis. Numerous indicators of the potential dependency of the beetle on this prey were investigated in three separate field study areas and in the laboratory. The distribution patterns and fluctuations of both groups were documented in detail in the field. The data were collected without interruption from 1992 to 2000 in seven ponds located in an agricultural landscape south of Bonn, Germany, and from 1995 to 1997 in nine ponds located in a forest west of Bonn. For this purpose, a surfacing trap was constructed as a method of live-trapping that is particularly easy on the environment and the animals. In order to statistically estimate beetle populations, a reliable mark-release-recapture method (MRR) adapted to the biology of D.°marginalis and the prevailing study conditions was developed for long-term estimates with small sample sizes. The distribution pattern observed in the first two study areas was re-examined in 1998 by additionally surveying corresponding populations in 26 small bodies of water located in three hill-country valleys 50 km away. Twenty-four aquariums were set up for comparative testing under controlled conditions. The development of small populations of D.°marginalis larvae exposed to different feeding resources was recorded. In addition, similarities and differences between the phenologies of different Dytiscus species and indigenous anurans were also analysed in these aquariums. The field surveys showed that the distribution pattern of the beetle was the same in all three study areas as that of the anuran species Bufo bufo, Rana temporaria and Rana dalmatina, which spawn early in the year. The synchronous distribution of the predator with the tadpoles of these three prey species, as well as the disappearance of the two during the course of the surveys, was documented in particularly great detail in the man-made ponds located in the agricultural region. The laboratory studies indicated that the common distribution patterns of these animal groups can be attributed to a direct dependence of the beetle larvae on the tadpoles of the respective species. In the aquariums, for example, the successful development of a population of Dytiscus larvae was consistently and exclusively dependent on the density of the amphibian larvae and self-regulated by the cannibalism of the beetle larvae. The precise adaptation of the development phases of D.°marginalis to the early-spawning anurans was observed in both the field and the laboratory, and could thus be described as a direct adaptation of the beetle. The extremely effective and sustained resource utilisation of the large diving beetle compared to interspecific food competitors was also documented. These, and all other indicators examined in this study, clearly show that, although D.°marginalis can utilise a broad food spectrum, its larval development is contingent upon the presence of amphibian larvae. The great success of this beetle species can be attributed to the fact that, unlike other species in the genus, it has specialised on widely distributed species of "early-spawning" anurans that are also very abundant. The large diving beetle has developed additional adaptation strategies for this purpose, several of which have been identified here

Triggering up states in all-to-all coupled neurons

Slow-wave sleep in mammalians is characterized by a change of large-scale cortical activity currently paraphrased as cortical Up/Down states. A recent experiment demonstrated a bistable collective behaviour in ferret slices, with the remarkable property that the Up states can be switched on and off with pulses, or excitations, of same polarity; whereby the effect of the second pulse significantly depends on the time interval between the pulses. Here we present a simple time discrete model of a neural network that exhibits this type of behaviour, as well as quantitatively reproduces the time-dependence found in the experiments.Comment: epl Europhysics Letters, accepted (2010