16 research outputs found
Dynamic Time Warping Averaging of Time Series Allows Faster and More Accurate Classification
Recent years have seen significant progress in improving both the efficiency and effectiveness of time series classification. However, because the best solution is typically the Nearest Neighbor algorithm with the relatively expensive Dynamic Time Warping as the distance measure, successful deployments on resource constrained devices remain elusive. Moreover, the recent explosion of interest in wearable devices, which typically have limited computational resources, has created a growing need for very efficient classification algorithms. A commonly used technique to glean the benefits of the Nearest Neighbor algorithm, without inheriting its undesirable time complexity, is to use the Nearest Centroid algorithm. However, because of the unique properties of (most) time series data, the centroid typically does not resemble any of the instances, an unintuitive and underappreciated fact. In this work we show that we can exploit a recent result to allow meaningful averaging of 'warped' times series, and that this result allows us to create ultra-efficient Nearest 'Centroid' classifiers that are at least as accurate as their more lethargic Nearest Neighbor cousins
Faster and more accurate classification of time series by exploiting a novel dynamic time warping averaging algorithm
A concerted research effort over the past two decades has heralded significant improvements in both the efficiency and effectiveness of time series classification. The consensus that has emerged in the community is that the best solution is a surprisingly simple one. In virtually all domains, the most accurate classifier is the nearest neighbor algorithm with dynamic time warping as the distance measure. The time complexity of dynamic time warping means that successful deployments on resource-constrained devices remain elusive. Moreover, the recent explosion of interest in wearable computing devices, which typically have limited computational resources, has greatly increased the need for very efficient classification algorithms. A classic technique to obtain the benefits of the nearest neighbor algorithm, without inheriting its undesirable time and space complexity, is to use the nearest centroid algorithm. Unfortunately, the unique properties of (most) time series data mean that the centroid typically does not resemble any of the instances, an unintuitive and underappreciated fact. In this paper we demonstrate that we can exploit a recent result by Petitjean et al. to allow meaningful averaging of “warped” time series, which then allows us to create super-efficient nearest “centroid” classifiers that are at least as accurate as their more computationally challenged nearest neighbor relatives. We demonstrate empirically the utility of our approach by comparing it to all the appropriate strawmen algorithms on the ubiquitous UCR Benchmarks and with a case study in supporting insect classification on resource-constrained sensors
A new comprehensive set of elemental abundances in DLAs III. Star formation histories
We obtained comprehensive sets of elemental abundances for eleven damped
Ly-alpha systems (DLAs) at z_DLA=1.7-2.5. In Paper I of this series, we showed
for three DLA galaxies that we can derive their star formation histories and
ages from a detailed comparison of their intrinsic abundance patterns with
chemical evolution models. We determine in this paper the star formation
properties of six additional DLA galaxies. The derived results confirm that no
single star formation history explains the diverse sets of abundance patterns
in DLAs. We demonstrate that the various star formation histories reproducing
the DLA abundance patterns are typical of local irregular, dwarf starburst and
quiescent spiral galaxies. Independent of the star formation history, the DLAs
have a common characteristic of being weak star forming galaxies; models with
high star formation efficiencies are ruled out. All the derived DLA star
formation rates per unit area are moderate or low, with values between -3.2 <
log SFR < -1.1 M_sol yr^{-1} kpc^{-2}. The DLA abundance patterns require a
large spread in ages ranging from 20 Myr up to 3 Gyr. The oldest DLA in our
sample is observed at z_DLA=1.864 with an age estimated to more than 3 Gyr; it
nicely indicates that galaxies were already forming at z_f>10. But, most of the
DLAs show ages much younger than that of the Universe at the epoch of
observation. Young galaxies thus seem to populate the high redshift Universe at
z>2, suggesting relatively low redshifts of formation (z~3) for most
high-redshift galaxies. The DLA star formation properties are compared with
those of other high-redshift galaxies identified in deep imaging surveys with
the aim of obtaining a global picture of high-redshift objects.Comment: 19 pages, 11 figures, Accepted for publication in A&
Evolution of chemical abundances in Seyfert galaxies
We computed the chemical evolution of spiral bulges hosting Seyfert nuclei,
based on updated chemical and spectro-photometrical evolution models for the
bulge of our Galaxy, made predictions about other quantities measured in
Seyferts, and modeled the photometry of local bulges. The chemical evolution
model contains detailed calculations of the Galactic potential and of the
feedback from the central supermassive black hole, and the spectro-photometric
model covers a wide range of stellar ages and metallicities. We followed the
evolution of bulges in the mass range 10^9 - 10^{11} Msun by scaling the star
formation efficiency and the bulge scalelength as in the inverse-wind scenario
for elliptical galaxies, and considering an Eddington limited accretion onto
the central supermassive black hole. We successfully reproduced the observed
black hole-host bulge mass relation. The observed nuclear bolometric luminosity
is reproduced only at high redshift or for the most massive bulges; in the
other cases, at z = 0 a rejuvenation mechanism is necessary. The black hole
feedback is in most cases not significant in triggering the galactic wind. The
observed high star formation rates and metal overabundances are easily
achieved, as well as the constancy of chemical abundances with redshift and the
bulge present-day colours. Those results are not affected if we vary the index
of the stellar IMF from x=0.95 to x=1.35; a steeper IMF is instead required in
order to reproduce the colour-magnitude relation and the present K-band
luminosity of the bulge.Comment: 17 pages, 15 figures, 3 tables, accepted for publication in A&