7,017 research outputs found
Simple and Nearly Optimal Polynomial Root-finding by Means of Root Radii Approximation
We propose a new simple but nearly optimal algorithm for the approximation of
all sufficiently well isolated complex roots and root clusters of a univariate
polynomial. Quite typically the known root-finders at first compute some crude
but reasonably good approximations to well-conditioned roots (that is, those
isolated from the other roots) and then refine the approximations very fast, by
using Boolean time which is nearly optimal, up to a polylogarithmic factor. By
combining and extending some old root-finding techniques, the geometry of the
complex plane, and randomized parametrization, we accelerate the initial stage
of obtaining crude to all well-conditioned simple and multiple roots as well as
isolated root clusters. Our algorithm performs this stage at a Boolean cost
dominated by the nearly optimal cost of subsequent refinement of these
approximations, which we can perform concurrently, with minimum processor
communication and synchronization. Our techniques are quite simple and
elementary; their power and application range may increase in their combination
with the known efficient root-finding methods.Comment: 12 pages, 1 figur
Multi-Wavelength Properties of Barred Galaxies in the Local Universe. I: Virgo Cluster
We study in detail how the barred galaxy fraction varies as a function of
luminosity, HI gas mass, morphology and color in the Virgo cluster in order to
provide a well defined, statistically robust measurement of the bar fraction in
the local universe spanning a wide range in luminosity (factor of ~100) and HI
gas mass. We combine multiple public data-sets (UKIDSS near-infrared imaging,
ALFALFA HI gas masses, GOLDMine photometry). After excluding highly inclined
systems, we define three samples where galaxies are selected by their B-band
luminosity, H-band luminosity, and HI gas mass. We visually assign bars using
the high resolution H-band imaging from UKIDSS. When all morphologies are
included, the barred fraction is ~17-24% while for morphologically selected
discs, we find that the barred fraction in Virgo is ~29-34%: it does not depend
strongly on how the sample is defined and does not show variations with
luminosity or HI gas mass. The barred fraction depends most strongly on the
morphological composition of the sample: when the disc populations are
separated into lenticulars (S0--S0/a), early-type spirals (Sa--Sb), and
late-type spirals (Sbc--Sm), we find that the early-type spirals have a higher
barred fraction (~45-50%) compared to the lenticulars and late-type spirals
(~22-36%). This difference may be due to the higher baryon fraction of
early-type discs which makes them more susceptible to bar instabilities. We do
not find any evidence of barred galaxies being preferentially blue.Comment: 13 pages, 14 figures. Submitted to Ap
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