289 research outputs found
Über die Eigenschaften von Schweissnähten niob-haltiger Zirkonium-Legierung. EUR 1319. = On the properties of the welding seams of niobium-containing zirconium alloy. EUR 1319.
Groups of Fibonacci type revisited
This article concerns a class of groups of Fibonacci type introduced by Johnson and Mawdesley that includes Conway?s Fibonacci groups, the Sieradski groups, and the Gilbert-Howie groups. This class of groups provides an interesting focus for developing the theory of cyclically presented groups and, following questions by Bardakov and Vesnin and by Cavicchioli, Hegenbarth, and Repov?s, they have enjoyed renewed interest in recent years. We survey results concerning their algebraic properties, such as isomorphisms within the class, the classification of the finite groups, small cancellation properties, abelianizations, asphericity, connections with Labelled Oriented Graph groups, and the semigroups of Fibonacci type. Further, we present a new method of proving the classification of the finite groups that deals with all but three groups
Optimal bounds for a colorful Tverberg--Vrecica type problem
We prove the following optimal colorful Tverberg-Vrecica type transversal
theorem: For prime r and for any k+1 colored collections of points C^l of size
|C^l|=(r-1)(d-k+1)+1 in R^d, where each C^l is a union of subsets (color
classes) C_i^l of size smaller than r, l=0,...,k, there are partition of the
collections C^l into colorful sets F_1^l,...,F_r^l such that there is a k-plane
that meets all the convex hulls conv(F_j^l), under the assumption that r(d-k)
is even or k=0.
Along the proof we obtain three results of independent interest: We present
two alternative proofs for the special case k=0 (our optimal colored Tverberg
theorem (2009)), calculate the cohomological index for joins of chessboard
complexes, and establish a new Borsuk-Ulam type theorem for (Z_p)^m-equivariant
bundles that generalizes results of Volovikov (1996) and Zivaljevic (1999).Comment: Substantially revised version: new notation, improved results,
additional references; 12 pages, 2 figure
Modeling of internal tides in fjords
A previous model for the distribution of internal tides above irregular topography is generalized to include arbitrary stratification and a radiation condition at the open boundary. Thanks to a small amount of dissipation, this model remains valid in the presence of resonant internal tides, leading to intense wave-energy beams. An application to a Norwegian fjord correctly reproduces the observed energy pattern consisting of two beams both originating at the 60-meter deep entrance sill and extending in-fjord, one upward toward the surface, the other downward toward the bottom. After correction for the varying width of the fjord, the observed and modelled energy levels are in good agreement, especially in the upper levels where energy is the greatest. Furthermore, the substantial phase lag between these two energy beams revealed by the observations is correctly reproduced by the model. Finally, a third and very narrow energy spike is noted in the model at the level of a secondary bump inward of the sill. This beam is missed by the current meter data, because the current meters were placed only at a few selected depths. But an examination of the salinity profiles reveals a mixed layer at approximately the same depth. The explanation is that high-wave energy leads to wave breaking and vigorous mixing. The model\u27s greatest advantage is to provide the internal-tide energy distribution throughout the fjord. Discrepancies between observations and model are attributed to coarse vertical resolution in the vicinity of the sill and to unaccounted cross-fjord variations
Resonance of internal waves in fjords: A finite-difference model
After the time periodicity is removed from the problem, the spatial distribution of internal waves in a stratified fluid is governed by a hyperbolic equation. With boundary conditions specified all along the perimeter of the domain, information is transmitted in both directions (forward and backward) along every characteristic, and, unlike the typical temporal hyperbolic equation, the internal-wave equation is not amenable to a simple forward integration. The problem is tackled here with a finite-difference, relaxation technique by constructing a time-dependent, dissipative problem, the final steady state of which yields the solution of the original problem. Attempts at solving the problem for arbitrary topography then reveal multiple resonances, each resonance being caused by a ray path closing onto itself after multiple reflections. The finite-difference formulation is found to be a convenient vehicle to discuss resonances and to conclude that their existence renders the problem not only singular but also extremely sensitive to the details of the topography. The problem is easily overcome by the introduction of friction. The finite-difference representation of the problem is instrumental in serving as a guide for the investigation of the resonance problem. Indeed, it keeps the essence of the continuous problem and yet simplifies the analysis enormously. Although straightforward, robust and successful at providing a numerical solution to a first few examples, the relaxation component of the integration technique suffers from lack of efficiency. This is due to the particular nature of the hyperbolic problem, but it remains that numerical analysts could improve or replace the present scheme with a faster algorithm
Mid-summer vertical behavior of a high-latitude oceanic zooplankton community
Vertical behavior, such as diel vertical migration (DVM) and swarming are widespread among zooplankton. At
higher latitudes, synchronized DVM is mostly absent during summer and predominantly herbivorous copepods
tend to form large near-surface swarms. This behavior is risky because it can make them vulnerable to visual
predators. Here, we used ca. 12 days of mid-summer (28 June to 10 July 2018) high-frequency acoustic data
collected on board of an autonomous surface vehicle (Sailbuoy) to study the vertical behavioral patterns of a
zooplankton community in the Norwegian Sea (69◦–71◦ N). Comparing acoustic data with zooplankton net
samples, we could distinguish the sound scatters into (1). lipid-rich older developmental stages of Calanus spp.,
(2). younger developmental stages of Calanus spp., smaller copepods and krill and (3). unknown group of strong
sound scatters that may have been younger stages of planktivorous fish. We observed shorter-range classic DVM
during much of the study period, where in two days, the migration appeared to be pronounced (> 50 m in
amplitude), largely synchronous and occurred in the presence of sound scatterer group 3. The observed
zooplankton community was concentrated in the upper 20 m in cloudy and calm days but retreated to greater
depths at increased near-surface turbulence. This turbulence-driven vertical retreat appeared to be synchronized
across the zooplankton community, potentially indicating a schooling behavior
A topological central point theorem
In this paper a generalized topological central point theorem is proved for
maps of a simplex to finite-dimensional metric spaces. Similar generalizations
of the Tverberg theorem are considered.Comment: In this version some typos were corrected after the official
publicatio
On the maximum size of an anti-chain of linearly separable sets and convex pseudo-discs
We show that the maximum cardinality of an anti-chain composed of
intersections of a given set of n points in the plane with half-planes is close
to quadratic in n. We approach this problem by establishing the equivalence
with the problem of the maximum monotone path in an arrangement of n lines. For
a related problem on antichains in families of convex pseudo-discs we can
establish the precise asymptotic bound: it is quadratic in n. The sets in such
a family are characterized as intersections of a given set of n points with
convex sets, such that the difference between the convex hulls of any two sets
is nonempty and connected.Comment: 10 pages, 3 figures. revised version correctly attributes the idea of
Section 3 to Tverberg; and replaced k-sets by "linearly separable sets" in
the paper and the title. Accepted for publication in Israel Journal of
Mathematic
A Tverberg type theorem for matroids
Let b(M) denote the maximal number of disjoint bases in a matroid M. It is
shown that if M is a matroid of rank d+1, then for any continuous map f from
the matroidal complex M into the d-dimensional Euclidean space there exist t
\geq \sqrt{b(M)}/4 disjoint independent sets \sigma_1,\ldots,\sigma_t \in M
such that \bigcap_{i=1}^t f(\sigma_i) \neq \emptyset.Comment: This article is due to be published in the collection of papers "A
Journey through Discrete Mathematics. A Tribute to Jiri Matousek" edited by
Martin Loebl, Jaroslav Nesetril and Robin Thomas, due to be published by
Springe
Untersuchung von Legierungen des Zirkoniums mit Niob und Zinn im Hinblick auf deren Verwendung als Hüllmaterial in wassergekühlten Kernreaktoren. EUR 86.= Examination of zirconium alloys with tin and niobium in terms of their use as a shell material in water-cooled nuclear reactors. EUR 86.
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