7,335 research outputs found
Approximation Algorithms for Multi-Criteria Traveling Salesman Problems
In multi-criteria optimization problems, several objective functions have to
be optimized. Since the different objective functions are usually in conflict
with each other, one cannot consider only one particular solution as the
optimal solution. Instead, the aim is to compute a so-called Pareto curve of
solutions. Since Pareto curves cannot be computed efficiently in general, we
have to be content with approximations to them.
We design a deterministic polynomial-time algorithm for multi-criteria
g-metric STSP that computes (min{1 +g, 2g^2/(2g^2 -2g +1)} + eps)-approximate
Pareto curves for all 1/2<=g<=1. In particular, we obtain a
(2+eps)-approximation for multi-criteria metric STSP. We also present two
randomized approximation algorithms for multi-criteria g-metric STSP that
achieve approximation ratios of (2g^3 +2g^2)/(3g^2 -2g +1) + eps and (1 +g)/(1
+3g -4g^2) + eps, respectively.
Moreover, we present randomized approximation algorithms for multi-criteria
g-metric ATSP (ratio 1/2 + g^3/(1 -3g^2) + eps) for g < 1/sqrt(3)), STSP with
weights 1 and 2 (ratio 4/3) and ATSP with weights 1 and 2 (ratio 3/2). To do
this, we design randomized approximation schemes for multi-criteria cycle cover
and graph factor problems.Comment: To appear in Algorithmica. A preliminary version has been presented
at the 4th Workshop on Approximation and Online Algorithms (WAOA 2006
A Statistical Semi-Empirical Model: Satellite galaxies in Groups and Clusters
We present STEEL a STatistical sEmi-Empirical modeL designed to probe the
distribution of satellite galaxies in groups and clusters. Our fast statistical
methodology relies on tracing the abundances of central and satellite haloes
via their mass functions at all cosmic epochs with virtually no limitation on
cosmic volume and mass resolution. From mean halo accretion histories and
subhalo mass functions the satellite mass function is progressively built in
time via abundance matching techniques constrained by number densities of
centrals in the local Universe. By enforcing dynamical merging timescales as
predicted by high-resolution N-body simulations, we obtain satellite
distributions as a function of stellar mass and halo mass consistent with
current data. We show that stellar stripping, star formation, and quenching
play all a secondary role in setting the number densities of massive satellites
above . We further show that observed
star formation rates used in our empirical model over predict low-mass
satellites below , whereas, star
formation rates derived from a continuity equation approach yield the correct
abundances similar to previous results for centrals.Comment: 21 pages, 17 Figures. MNRAS, in pres
How do employees perceive the impact of health and safety policies on logistics delivery performance? A case study of a cement manufacturer based in Malaysia
Objective: The study explores the perception of the impact of health and safety policies on the delivery performance of a cement manufacturer in Malaysia. Methods: Data was collected by adopting a case study methodology approach which involved interviewing staff at senior (2), mid (3) and operational (2) levels followed by a survey of 48 truck drivers, 8 observations and the intranet data. Results: Senior Management believe that a rigorous adoption of health and safety policies will result in long term competitiveness and beneficial for the organisation as a whole whereas operations, mid-level management and truck drivers believe that the health and safety policies are restricting delivery flexibility and responsiveness to customer’s requirement. Health and Safety policies can have a negative impact on delivery flexibility on short term basis but in the long term it has reduced the time lost due to injuries which will positively impact delivery performance. Practical applications: The findings of this study will enable organisations to gain a deeper understanding about the impact of health and safety policies on delivery performance
Super-massive Black Hole Demography: the Match between the Local and Accreted Mass Functions
We have performed a detailed analysis of the local super-massive black-hole
(SMBH) mass function based on both kinematic and photometric data and derived
an accurate analytical fit in the range 10^6 <= (M_BH/M_sun) <= 5*10^9. We find
a total SMBH mass density of (4.2+/-1.1)*10^5 M_sun/Mpc^3, about 25% of which
is contributed by SMBHs residing in bulges of late type galaxies. Exploiting
up-to-date luminosity functions of hard X-ray and optically selected AGNs, we
have studied the accretion history of the SMBH population. If most of the
accretion happens at constant \dot{M_BH}/M_BH the local SMBH mass function is
fully accounted for by mass accreted by X-ray selected AGNs, with bolometric
corrections indicated by current observations and a standard mass-to-light
conversion efficiency \epsilon ~10%. The analysis of the accretion history
highlights that the most massive BHs accreted their mass faster and at higher
redshifts (z>1.5), while the lower mass BHs responsible for most of the hard
X-ray background have mostly grown at z<1.5. The accreted mass function matches
the local SMBH mass function if \epsilon ~0.09(+0.04,-0.03) and the Eddington
ratio \lambda=L/L_Edd \~0.3(+0.3,-0.1) (68% confidence errors). The visibility
time, during which AGNs are luminous enough to be detected by the currently
available X-ray surveys, ranges from ~0.1 Gyr for present day BH masses
M_BH(z=0) ~10^6 M_sun to ~0.3 Gyr for M_BH(z=0) >= 10^9 M_sun. The mass
accreted during luminous phases is >= 25-30% even if we assume extreme values
of \epsilon (\epsilon \~0.3-0.4). An unlikely fine tuning of the parameters
would be required to account for the local SMBH mass function accomodating a
dominant contribution from 'dark' BH growth (due, e.g., to BH coalescence).Comment: 12 pages, 14 figures, accepted for publication in MNRAS, minor
changes following referee's comment
Thermodynamics and kinetics of reactions in protective coating systems
A study of the aluminization of Ni from packs containing various percentages of unalloyed Al confirmed that the surface aluminum content of specimens aluminized tends to decrease with time and consequently a simple parabolic law for the weight-gain vs. time relationship is not obeyed. The diffusivity-composition relationship in NiAl was examined, and a set of curves is presented. A numerical method for the calculation of coating dissolution rates was developed and applied to NiAl-Ni3Al type of coatings
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Passive wireless antenna sensor for strain, temperature, crack and fatigue measurement
An apparatus and method is provided for monitoring a condition of a structure using a passive wireless antenna sensor having a known resonant frequency when mounted on the structure. A series of radio frequency signals are transmitted with sweeping frequencies around the known resonant frequency to the passive wireless antenna sensor. The passive wireless antenna sensor includes a dielectric substrate disposed between an antenna pattern and a ground plane such that a change in the condition of the structure will cause a change in one or more characteristics of the passive wireless sensor. A signal is received from the passive wireless antenna sensor and a resonant frequency of the passive wireless antenna sensor is determined based on the received signal. The determined resonant frequency is then compared to the known resonant frequency, whereby a change in the resonant frequency indicates a change in the condition of the structure.Board of Regents, University of Texas Syste
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