14,511 research outputs found
The growth of a Super Stable Heap : an experimental and numerical study
We report experimental and numerical results on the growth of a super stable
heap (SSH). Such a regime appears for flows in a thin channel and for high flow
rate : the flow occurs atop a nearly static heap whose angle is stabilized by
the flowing layer at its top and the side wall friction. The growth of the
static heap is investigated in this paper. A theoretical analysis inspired by
the BRCE formalism predicts the evolution of the growth process, which is
confirmed by both experiments and numerical simulations. The model allows us to
link the characteristic time of the growth to the exchange rate between the
"moving" and "static" grains. We show that this rate is proportional to the
height of the flowing layer even for thick flows. The study of upstream
traveling waves sheds new light on the BCRE model
A Back-to-Basics Empirical Study of Priority Queues
The theory community has proposed several new heap variants in the recent
past which have remained largely untested experimentally. We take the field
back to the drawing board, with straightforward implementations of both classic
and novel structures using only standard, well-known optimizations. We study
the behavior of each structure on a variety of inputs, including artificial
workloads, workloads generated by running algorithms on real map data, and
workloads from a discrete event simulator used in recent systems networking
research. We provide observations about which characteristics are most
correlated to performance. For example, we find that the L1 cache miss rate
appears to be strongly correlated with wallclock time. We also provide
observations about how the input sequence affects the relative performance of
the different heap variants. For example, we show (both theoretically and in
practice) that certain random insertion-deletion sequences are degenerate and
can lead to misleading results. Overall, our findings suggest that while the
conventional wisdom holds in some cases, it is sorely mistaken in others
The engineering geology of the Nottingham area, UK
Nottingham was built near a crossing point on the River Trent in the East Midlands of England. Initially, the City developed on a low sandstone hill close to the north bank of the river, which provided a secure, well-drained location above the marshes that bordered the river. Geologically, Nottingham stands at the boundary between Palaeozoic rocks to the north and west, and Mesozoic and Cainozoic strata to the south and east. The area is underlain by coal-bearing Carboniferous Coal Measures, Permian dolomitic limestones, Permo-Triassic mudstones and weak sandstones, Jurassic clays and Quaternary glacial and alluvial deposits. Artificial deposits, resulting from the social, industrial and mineral extraction activities of the past cover the natural deposits over much of the area.
This geological environment has underpinned the economic development of the area through the mining of coal (now largely ceased), oil extraction that was important during the Second World War, brickmaking from clays, alluvial sand and gravel extraction from the Trent Valley and gypsum extraction from the Permo-Triassic mudstones. The Permo-Triassic sandstone is a nationally important aquifer and has also been exploited at the surface and from shallow mines for sand.
However, this history of the use and exploitation of mineral deposits has created a number of environmental problems including rising groundwater levels, abandoned mine shafts and mining subsidence, and, within the City itself, the occasional collapse of artificial cavities in the sandstone and contaminated land left by industrial activities. Natural constraints on development include gypsum dissolution, landslides, rockfalls, swell-shrink problems in Jurassic clays and flooding. Occasional minor earthquakes are attributed to movements due to coal mining or natural, deep geological structures. Thus, Nottingham’s geological context remains an important consideration when planning its future regeneration and development
Zipf and Heaps laws from dependency structures in component systems
Complex natural and technological systems can be considered, on a
coarse-grained level, as assemblies of elementary components: for example,
genomes as sets of genes, or texts as sets of words. On one hand, the joint
occurrence of components emerges from architectural and specific constraints in
such systems. On the other hand, general regularities may unify different
systems, such as the broadly studied Zipf and Heaps laws, respectively
concerning the distribution of component frequencies and their number as a
function of system size. Dependency structures (i.e., directed networks
encoding the dependency relations between the components in a system) were
proposed recently as a possible organizing principles underlying some of the
regularities observed. However, the consequences of this assumption were
explored only in binary component systems, where solely the presence or absence
of components is considered, and multiple copies of the same component are not
allowed. Here, we consider a simple model that generates, from a given ensemble
of dependency structures, a statistical ensemble of sets of components,
allowing for components to appear with any multiplicity. Our model is a minimal
extension that is memoryless, and therefore accessible to analytical
calculations. A mean-field analytical approach (analogous to the "Zipfian
ensemble" in the linguistics literature) captures the relevant laws describing
the component statistics as we show by comparison with numerical computations.
In particular, we recover a power-law Zipf rank plot, with a set of core
components, and a Heaps law displaying three consecutive regimes (linear,
sub-linear and saturating) that we characterize quantitatively
Why some heaps support constant-amortized-time decrease-key operations, and others do not
A lower bound is presented which shows that a class of heap algorithms in the
pointer model with only heap pointers must spend Omega(log log n / log log log
n) amortized time on the decrease-key operation (given O(log n) amortized-time
extract-min). Intuitively, this bound shows the key to having O(1)-time
decrease-key is the ability to sort O(log n) items in O(log n) time; Fibonacci
heaps [M.L. Fredman and R. E. Tarjan. J. ACM 34(3):596-615 (1987)] do this
through the use of bucket sort. Our lower bound also holds no matter how much
data is augmented; this is in contrast to the lower bound of Fredman [J. ACM
46(4):473-501 (1999)] who showed a tradeoff between the number of augmented
bits and the amortized cost of decrease-key. A new heap data structure, the
sort heap, is presented. This heap is a simplification of the heap of Elmasry
[SODA 2009: 471-476] and shares with it a O(log log n) amortized-time
decrease-key, but with a straightforward implementation such that our lower
bound holds. Thus a natural model is presented for a pointer-based heap such
that the amortized runtime of a self-adjusting structure and amortized lower
asymptotic bounds for decrease-key differ by but a O(log log log n) factor
THE EFFECTS OF ISOPROPYL N-PHENYL CARBAMATE ON THE GREEN ALGA OEDOGONIUM CARDIACUM : I. Cell Division
Cell division in vegetative filaments of the green alga Oedogonium cardiacum is presented as an experimental system. We report on how we have used this system to study the effects of isopropyl N-phenylcarbamate (IPC) on the mitotic apparatus and on the phycoplast, a planar array of cytokinetic microtubules. Polymerization of microtubules was prevented when filaments, synchronized by a light/dark regime and chilled (2°C) while in metaphase or just before phycoplast formation, were exposed to 5.5 x 10-4 M IPC and then returned to room temperature. Spindles reformed or phycoplasts formed when these filaments were transferred to growth medium free of IPC. However, the orientation of both microtubular systems was disturbed: the mitotic apparatus often contained three poles, frequently forming three daughter nuclei upon karyokinesis; the phycoplast was often stellate rather than planar, and it sometimes was displaced to the side of both daughter nuclei, resulting in a binucleate and an anucleate cell upon cytokinesis. Our results suggest that IPC (a) prevents the assembly of microtubules, (b) increases the number of functional polar bodies, and (c) affects the orientation of microtubules in O. cardiacum. High voltage (1,000 kV) electron microscopy of 0.5-µm thick sections allowed us to visualize the polar structures, which were not discernible in thin sections
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