570,633 research outputs found
Route Planning in Transportation Networks
We survey recent advances in algorithms for route planning in transportation
networks. For road networks, we show that one can compute driving directions in
milliseconds or less even at continental scale. A variety of techniques provide
different trade-offs between preprocessing effort, space requirements, and
query time. Some algorithms can answer queries in a fraction of a microsecond,
while others can deal efficiently with real-time traffic. Journey planning on
public transportation systems, although conceptually similar, is a
significantly harder problem due to its inherent time-dependent and
multicriteria nature. Although exact algorithms are fast enough for interactive
queries on metropolitan transit systems, dealing with continent-sized instances
requires simplifications or heavy preprocessing. The multimodal route planning
problem, which seeks journeys combining schedule-based transportation (buses,
trains) with unrestricted modes (walking, driving), is even harder, relying on
approximate solutions even for metropolitan inputs.Comment: This is an updated version of the technical report MSR-TR-2014-4,
previously published by Microsoft Research. This work was mostly done while
the authors Daniel Delling, Andrew Goldberg, and Renato F. Werneck were at
Microsoft Research Silicon Valle
Computing generators of the unit group of an integral abelian group ring
We describe an algorithm for obtaining generators of the unit group of the
integral group ring ZG of a finite abelian group G. We used our implementation
in Magma of this algorithm to compute the unit groups of ZG for G of order up
to 110. In particular for those cases we obtained the index of the group of
Hoechsmann units in the full unit group. At the end of the paper we describe an
algorithm for the more general problem of finding generators of an arithmetic
group corresponding to a diagonalizable algebraic group
Simultaneous whole-animal 3D-imaging of neuronal activity using light field microscopy
3D functional imaging of neuronal activity in entire organisms at single cell
level and physiologically relevant time scales faces major obstacles due to
trade-offs between the size of the imaged volumes, and spatial and temporal
resolution. Here, using light-field microscopy in combination with 3D
deconvolution, we demonstrate intrinsically simultaneous volumetric functional
imaging of neuronal population activity at single neuron resolution for an
entire organism, the nematode Caenorhabditis elegans. The simplicity of our
technique and possibility of the integration into epi-fluoresence microscopes
makes it an attractive tool for high-speed volumetric calcium imaging.Comment: 25 pages, 7 figures, incl. supplementary informatio
Probing the nature of dark energy through galaxy redshift surveys with radio telescopes
Galaxy redshift surveys using optical telescopes have, in combination with
other cosmological probes, enabled precision measurements of the nature of dark
energy. We show that radio telescopes are rapidly becoming competitive with
optical facilities in spectroscopic surveys of large numbers of galaxies. Two
breakthroughs are driving this change. Firstly, individual radio telescopes are
more efficient at mapping the sky thanks to the large field-of-view of new
phased-array feeds. Secondly, ever more dishes can be correlated in a
cost-effective manner with rapid increases in computing power. The next decade
will see the coming of age of the 21cm radio wavelength as a cosmological probe
as first the Pathfinders then, ultimately, the Square Kilometre Array is
constructed. The latter will determine precise 3D positions for a billion
galaxies, mapping the distribution of matter in the Universe over the last 12
billion years. This radio telescope will be able to constrain the equation of
state of dark energy, and its potential evolution, to a precision rivalling
that of future optical facilities such as DESI and Euclid.Comment: 17 pages, 2 figures, 1 table. Accepted to Annalen der Physik for the
Special Issue "The Accelerating Universe
Simulation and static measurement of the gas volume fraction in a separated flow model using a Conductance Multiphase Venturi Meter (CMVM)
The assumption of equal mixture densities at the inlet and the throat of the Venturi is only valid in a homogenous flow. This assumption is no longer valid for a separated flow and therefore, the measurement of the gas volume fraction at the throat must be introduced. This paper presents an advanced Conductance Multiphase Venturi Meter (CMVM) which is capable of measuring the gas volume fraction at the throat of the CMVM. In water continuous multiphase flow, the electrical conductance technique has proven attractive for many industrial applications. In gas-water two phase flow the electric conductance technique can be used to extract the gas volume fraction. We measure conductance using two ring electrodes flush with the inner surface of the Venturi throat. Different nylon rods with different diameters were inserted at the center of the throat to simulate the gas volume fraction at the throat. Experimental static tests for a vertical and a horizontal Venturi meter were performed in which the water film thickness and the gas volume fraction (i.e. the volume occupied by a non-conducting rod) were measured. In this paper, the experimental results were also compared with those obtained from COMSOL finite element software. It was inferred from this comparison that the error was less than 1.95 %
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