6,025 research outputs found
GreenPhylDB v2.0: An improved database for plant functional genomics
Poster presented at 2009 Annual Research Meeting of the Generation Challenge Programme. Bamako (Mali), 20-23 September 200
Locating Depots for Capacitated Vehicle Routing
We study a location-routing problem in the context of capacitated vehicle
routing. The input is a set of demand locations in a metric space and a fleet
of k vehicles each of capacity Q. The objective is to locate k depots, one for
each vehicle, and compute routes for the vehicles so that all demands are
satisfied and the total cost is minimized. Our main result is a constant-factor
approximation algorithm for this problem. To achieve this result, we reduce to
the k-median-forest problem, which generalizes both k-median and minimum
spanning tree, and which might be of independent interest. We give a
(3+c)-approximation algorithm for k-median-forest, which leads to a
(12+c)-approximation algorithm for the above location-routing problem, for any
constant c>0. The algorithm for k-median-forest is just t-swap local search,
and we prove that it has locality gap 3+2/t; this generalizes the corresponding
result known for k-median. Finally we consider the "non-uniform"
k-median-forest problem which has different cost functions for the MST and
k-median parts. We show that the locality gap for this problem is unbounded
even under multi-swaps, which contrasts with the uniform case. Nevertheless, we
obtain a constant-factor approximation algorithm, using an LP based approach.Comment: 12 pages, 1 figur
A re-interpretation of the Triangulum-Andromeda stellar clouds: a population of halo stars kicked out of the Galactic disk
The Triangulum-Andromeda stellar clouds (TriAnd1 and TriAnd2) are a pair of
concentric ring- or shell-like over-densities at large ( 30 kpc)
and ( -10 kpc) in the Galactic halo that are thought to have been
formed from the accretion and disruption of a satellite galaxy. This paper
critically re-examines this formation scenario by comparing the number ratio of
RR Lyrae to M giant stars associated with the TriAnd clouds with other
structures in the Galaxy. The current data suggest a stellar population for
these over-densities ( at 95% confidence) quite unlike
any of the known satellites of the Milky Way ( for
the very largest and for the smaller satellites) and more
like the population of stars born in the much deeper potential well inhabited
by the Galactic disk (). N-body simulations of a
Milky-Way-like galaxy perturbed by the impact of a dwarf galaxy demonstrate
that, in the right circumstances, concentric rings propagating outwards from
that Galactic disk can plausibly produce similar over-densities. These results
provide dramatic support for the recent proposal by Xu et al. (2015) that,
rather than stars accreted from other galaxies, the TriAnd clouds could
represent stars kicked-out from our own disk. If so, these would be the first
populations of disk stars to be found in the Galactic halo and a clear
signature of the importance of this second formation mechanism for stellar
halos more generally. Moreover, their existence at the very extremities of the
disk places strong constraints on the nature of the interaction that formed
them.Comment: 27 pages, 8 figures; published in MNRA
A real delivery problem dealt with Monte Carlo techniques
[EN] In this paper we use Monte Carlo Techniques to deal with a real world delivery problem of a food company in Valencia (Spain). The problem is modeled as a set of 11 instances of the well known Vehicle Routing Problem, VRP, with additional time constraints. Given that VRP is a NP-hard problem, a heuristic algorithm, based on Monte Carlo techniques, is implemented. The solution proposed by this heuristic algorithm reaches distance and money savings of about 20% and 5% respectively.This research was partially supported by MICINN, Project MTM2013-43540-P and by UPV, Project Programa de Apoyo a la Investigación y Desarrollo de la UPV PAID-06-12.S577181Fernández de Córdoba, P., L.M. García-Raffi and J.M. Sanchis Llopis (1998), A heuristic algorithm based on Monte Carlo methods for the Rural Postman Problem.Computers and Op. Research,25, No. 12, pp. 1097–1106, 1998.Fernández de Córdoba, P. and L.M. García-Raffi, E. Nieto and J.M. Sanchis Llopis (1999a), Aplicación de técnicas Monte Carlo a un problema real de Rutas de Vehículos.Anales de Ingeniería, Colombia. In press.Fernández de Córdoba, P., L.M. García-Raffi and J.M. Sanchis Llopis (1999b), A Constructive Parallel Algorithm based on Monte Carlo techniques for Routing Problems, Submitted toParallel Computers.Laporte, G. (1992), The Vehicle Routing Problem: an overview of exact and approximate algorithms,European Journal of Operations Research,59, 345.Laporte, G., M. Desrochers and Y. Nobert (1985), “Optimal Routing under Capacity and Distance Restrictions.Operations Research,33, pp. 1050–1073.Laporte G. and Y. Nobert (1987), Exact algorithms for The Vehicle Routing Problem,Surveys in Combinatorial Optimization (S. Martello, G. Laporte, M. Minoux and C. Ribeiro Eds.). North-HollandAmsterdamMayado, A. (1998), Organización de los itinerarios de la flota de camiones de reparto de una sociedad cooperativa. Optimización mediante técnicas de simulación Monte Carlo. Proyecto Fin de Carrera. E.T.S.I.I. Universidad Politécnica de Valencia
Disk Heating, Galactoseismology, and the Formation of Stellar Halos
Deep photometric surveys of the Milky Way have revealed diffuse structures
encircling our Galaxy far beyond the "classical" limits of the stellar disk.
This paper reviews results from our own and other observational programs, which
together suggest that, despite their extreme positions, the stars in these
structures were formed in our Galactic disk. Mounting evidence from recent
observations and simulations implies kinematic connections between several of
these distinct structures. This suggests the existence of collective disk
oscillations that can plausibly be traced all the way to asymmetries seen in
the stellar velocity distribution around the Sun. There are multiple
interesting implications of these findings: they promise new perspectives on
the process of disk heating, they provide direct evidence for a stellar halo
formation mechanism in addition to the accretion and disruption of satellite
galaxies, and, they motivate searches of current and near-future surveys to
trace these oscillations across the Galaxy. Such maps could be used as
dynamical diagnostics in the emerging field of "Galactoseismology", which
promises to model the history of interactions between the Milky Way and its
entourage of satellites, as well examine the density of our dark matter halo.
As sensitivity to very low surface brightness features around external galaxies
increases, many more examples of such disk oscillations will likely be
identified. Statistical samples of such features not only encode detailed
information about interaction rates and mergers, but also about long
sought-after dark matter halo densities and shapes. Models for the Milky Way's
own Galactoseismic history will therefore serve as a critical foundation for
studying the weak dynamical interactions of galaxies across the universe.Comment: 20 pages, 5 figures, accepted in for publication in a special edition
of the journal "Galaxies", reporting the proceedings of the conference "On
the Origin (and Evolution) of Baryonic Galaxy Halos", Puerto Ayora, Ecuador,
March 13-17 2017, Eds. Duncan A. Forbes and Ericson D. Lope
Multiwavelength characterisation of an ACT-selected, lensed dusty star-forming galaxy at z=2.64
We present \ci\,(2--1) and multi-transition CO observations of a dusty
star-forming galaxy, ACT\,J2029+0120, which we spectroscopically confirm to lie
at \,=\,2.64. We detect CO(3--2), CO(5--4), CO(7--6), CO(8--7), and
\ci\,(2--1) at high significance, tentatively detect HCO(4--3), and place
strong upper limits on the integrated strength of dense gas tracers (HCN(4--3)
and CS(7--6)). Multi-transition CO observations and dense gas tracers can
provide valuable constraints on the molecular gas content and excitation
conditions in high-redshift galaxies. We therefore use this unique data set to
construct a CO spectral line energy distribution (SLED) of the source, which is
most consistent with that of a ULIRG/Seyfert or QSO host object in the taxonomy
of the \textit{Herschel} Comprehensive ULIRG Emission Survey. We employ RADEX
models to fit the peak of the CO SLED, inferring a temperature of T117 K
and cm, most consistent with a ULIRG/QSO object
and the presence of high density tracers. We also find that the velocity width
of the \ci\ line is potentially larger than seen in all CO transitions for this
object, and that the ratio is also larger
than seen in other lensed and unlensed submillimeter galaxies and QSO hosts; if
confirmed, this anomaly could be an effect of differential lensing of a shocked
molecular outflow.Comment: Accepted for publication in Ap
The ALMA Frontier Fields Survey - IV. Lensing-corrected 1.1 mm number counts in Abell 2744, MACSJ0416.1-2403 and MACSJ1149.5+2223
[abridged] Characterizing the number counts of faint, dusty star-forming
galaxies is currently a challenge even for deep, high-resolution observations
in the FIR-to-mm regime. They are predicted to account for approximately half
of the total extragalactic background light at those wavelengths. Searching for
dusty star-forming galaxies behind massive galaxy clusters benefits from strong
lensing, enhancing their measured emission while increasing spatial resolution.
Derived number counts depend, however, on mass reconstruction models that
properly constrain these clusters. We estimate the 1.1 mm number counts along
the line of sight of three galaxy clusters, i.e. Abell 2744, MACSJ0416.1-2403
and MACSJ1149.5+2223, which are part of the ALMA Frontier Fields Survey. We
perform detailed simulations to correct these counts for lensing effects. We
use several publicly available lensing models for the galaxy clusters to derive
the intrinsic flux densities of our sources. We perform Monte Carlo simulations
of the number counts for a detailed treatment of the uncertainties in the
magnifications and adopted source redshifts. We find an overall agreement among
the number counts derived for the different lens models, despite their
systematic variations regarding source magnifications and effective areas. Our
number counts span ~2.5 dex in demagnified flux density, from several mJy down
to tens of uJy. Our number counts are consistent with recent estimates from
deep ALMA observations at a 3 level. Below 0.1 mJy, however,
our cumulative counts are lower by 1 dex, suggesting a flattening in
the number counts. In our deepest ALMA mosaic, we estimate number counts for
intrinsic flux densities 4 times fainter than the rms level. This
highlights the potential of probing the sub-10 uJy population in larger samples
of galaxy cluster fields with deeper ALMA observations.Comment: 19 pages, 14 figures, 3 tables. Accepted for publication in A&
The Railway Line Frequency and Size Setting Problem
[EN] The problem studied in this paper takes as input data a set of lines
forming a railway network, and an origin¿destination (OD) matrix. The OD pairs
may use either the railway network or an alternative transportation mode. The
objective is to determine the frequency/headway of each line as well as its number
of carriages, so that the net profit of the railway network is maximized. We propose
a mixed integer non-linear programming formulation for this problem. Because of
the computational intractability of this model, we develop four algorithms: a mixed
integer linear programming (MIP) model, a MIP-based iterative algorithm, a
shortest-path based algorithm, and a local search. These four algorithms are tested
and compared over a set of randomly generated instances. An application over a
case study shows that only the local search heuristic is capable of dealing with large
instances.This research was partly funded by the Canadian Natural Sciences and Engineering Research Council under Grant 2015-06189, by the Ministerio de Economía y Competitividad (Spain)/FEDER under projects MTM2012-37048, MTM2015-67706-P and DPI2012-36243-C02-01, and by Junta de Andalucía (Spain)/FEDER under excellence project P10-FQM-5849. Part of this research was done while Federico Perea was enjoying a research visit to CIRRELT, funded by the Universitat Politècnica de València, under program PAID-00-15. This support is gratefully acknowledged. Thanks are due to the referees for their valuable comments.De-Los-Santos, A.; Laporte, G.; Mesa, JA.; Perea Rojas Marcos, F. (2017). The Railway Line Frequency and Size Setting Problem. Public Transport. 9(1-2):33-53. https://doi.org/10.1007/s12469-017-0154-2S335391-2Albrecht T (2009) Automated timetable design for demand-oriented service on suburban railways. Public Transport 1(1):5–20Caprara A, Kroon L, Monaci M, Peeters M, Toth P (2007) Passenger Railway optimization. In: Barnhart C, Laporte G (eds) Handbooks in operations research and management science, vol 14. Transportation, chapter 3. North-Holland, Amsterdam, pp 129–187De-Los-Santos A, Laporte G, Mesa J, Perea F (2014) Simultaneous frequency and capacity setting in uncapacitated metro lines in presence of a competing mode. Transp Res Proc 3:289–298Desaulniers G, Hickman M (2007) Public transport. In: Barnhart C, Laporte G (eds) Handbook in operations research and management science, vol 14, Transportation, chapter 2. North-Holland, Amsterdam, pp 69–127Gallo M, Montella B, D’Acierno L (2011) The transit network design problem with elastic demand and internalisation of external costs: An application to rail frequency optimisation. Transp Res Part C Emerg Technol 19(6):1276–1305Laporte G, Marín A, Mesa JA, Perea F (2011) Designing robust rapid transit networks with alternative routes. J Adv Transp 45(1):54–65Marín A, García-Ródenas R (2009) Location of infrastructure in urban railway networks. Comput Oper Res 36(5):1461–1477Michaelis M, Schöbel A (2009) Integrating line planning, timetable, and vehicle scheduling: a customer oriented heuristic. Public Transport 1(3):211–232Perea F, Mesa JA, Laporte G (2014) Adding a new station and a road link to a road-rail network in the presence of modal competition. Transp Res Part B Methodol 68:1–16Schmidt M, Schöbel A (2015) The complexity of integrating passenger routing decisions in public transportation models. Networks 65(3):228–243Schmidt ME (2014) Integrating routing decisions in public transportation problems. Springer, New YorkSchöbel A (2012) Line planning in public transportation. OR Spectrum 34:491–510van Oort N, van Nes R (2009) Regularity analysis for optimizing urban transit network design. Public Transport 1(2):155–168Vuchic VR (2005) Urban transit: operations, planning, and economics. Wiley, Hoboken, New Jerse
- …