2,685 research outputs found
Time-Optimal Tree Computations on Sparse Meshes
The main goal of this work is to fathom the suitability of the mesh with multiple broadcasting architecture (MMB) for some tree-related computations. We view our contribution at two levels: on the one hand, we exhibit time lower bounds for a number of tree-related problems on the MMB. On the other hand, we show that these lower bounds are tight by exhibiting time-optimal tree algorithms on the MMB. Specifically, we show that the task of encoding and/or decoding n-node binary and ordered trees cannot be solved faster than Ω(log n) time even if the MMB has an infinite number of processors. We then go on to show that this lower bound is tight. We also show that the task of reconstructing n-node binary trees and ordered trees from their traversais can be performed in O(1) time on the same architecture. Our algorithms rely on novel time-optimal algorithms on sequences of parentheses that we also develop
Straits and seaways: controls, processes and implications in modern and ancient systems
Straits and seaways are connections between basins, key areas for exchange and transfer of water, heat, biota and sediments, and they can influence regional and global climate. A better understanding of strait/seaway dynamics, their evolution and preserved deposits is, therefore, fundamental to reconstructing the palaeoecology, palaeogeography and stratigraphy of interconnected basins. Straits and seaways are also important for understanding climate dynamics in Earth's past, and for safely exploiting energy resources (fossil and renewable). This volume aims at improving the knowledge on this topic, and at providing a comprehensive collection of state-of-the-art works bringing together complementary disciplines. Even though studies on modern and ancient straits and seaways exist, they are not necessarily conceived towards a better geological understanding of these features as depositional systems in their own right. In this introduction, we emphasize the geological importance of straits and seaways, summarizing the content and key findings of the contributions on this topic. The articles included in this volume explore four main research themes related to straits and seaways: (1) occurrence and classification; (2) morphological features, facies and stratigraphic variability, sedimentary processes and dynamics; (3) tectonic and climatic controls, their feedback with climate changes; and (4) palaeogeographical reconstructions and preservation of associated deposits
Aharonov-Bohm interference in the presence of metallic mesoscopic cylinders
This work studies the interference of electrons in the presence of a line of
magnetic flux surrounded by a normal-conducting mesoscopic cylinder at low
temperature. It is found that, while there is a supplementary phase
contribution from each electron of the mesoscopic cylinder, the sum of these
individual supplementary phases is equal to zero, so that the presence of a
normal-conducting mesoscopic ring at low temperature does not change the
Aharonov-Bohm interference pattern of the incident electron. It is shown that
it is not possible to ascertain by experimental observation that the shielding
electrons have responded to the field of an incident electron, and at the same
time to preserve the interference pattern of the incident electron. It is also
shown that the measuring of the transient magnetic field in the region between
the two paths of an electron interference experiment with an accuracy at least
equal to the magnetic field of the incident electron generates a phase
uncertainty which destroys the interference pattern.Comment: 15 pages, 5 Postscript figure
A proposal for detecting second order topological quantum phase
Gaussian linking of a semiclassical path of a charged particle with a
magnetic flux tube is responsible for the Aharonov-Bohm effect, where one
observes interference proportional to the magnitude of the enclosed flux. We
construct quantum mechanical wave functions where semiclassical paths can have
second order linking to two magnetic flux tubes, and show there is interference
proportional to the product of the two fluxes.Comment: 7 pages, 4 figure
Perturbative Study of Bremsstrahlung and Pair-Production by Spin-1/2 Particles in the Aharonov-Bohm Potential
In the presence of an external Aharonov-Bohm potential, we investigate the
two QED processes of the emission of a bremsstrahlung photon by an electron,
and the production of an electron-positron pair by a single photon.
Calculations are carried out using the Born approximation within the framework
of covariant perturbation theory to lowest non-vanishing order in \alpha. The
matrix element for each process is derived, and the corresponding differential
cross-section is calculated. In the non-relativistic limit, the resulting
angular and spectral distributions and some polarization properties are
considered, and compared to results of previous works.Comment: 15 pages, RevTex 4, 2 figures, submitted for publicatio
Electron-positron pair production in the Aharonov-Bohm potential
In the framework of QED we evaluate the cross section for electron-positron
pair production by a single photon in the presence of the external
Aharonov-Bohm potential in first order of perturbation theory. We analyse
energy, angular and polarization distributions at different energy regimes:
near the threshold and at high photon energies.Comment: LaTeX file, 13 page
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