562 research outputs found
A new nascent spreading centre at the Wagner Basin in the northern Gulf of California: a possible geothermal resource?
The probable geothermal reserves of Mexico sum up to only 1400 MW; however, they have been estimated on the basis of the high temperature systems and do not include the unconventional geothermal sources. Submarine hydrothermal systems may become in the near future a feasible energy source, especially those that occur at shallow depths. Recently discovered hydrothermal activity in the Wagner Basin may be harnessed to produce electricity using an environmentally friendly system
Mirror-Descent Methods in Mixed-Integer Convex Optimization
In this paper, we address the problem of minimizing a convex function f over
a convex set, with the extra constraint that some variables must be integer.
This problem, even when f is a piecewise linear function, is NP-hard. We study
an algorithmic approach to this problem, postponing its hardness to the
realization of an oracle. If this oracle can be realized in polynomial time,
then the problem can be solved in polynomial time as well. For problems with
two integer variables, we show that the oracle can be implemented efficiently,
that is, in O(ln(B)) approximate minimizations of f over the continuous
variables, where B is a known bound on the absolute value of the integer
variables.Our algorithm can be adapted to find the second best point of a
purely integer convex optimization problem in two dimensions, and more
generally its k-th best point. This observation allows us to formulate a
finite-time algorithm for mixed-integer convex optimization
Statistically optimal analysis of state-discretized trajectory data from multiple thermodynamic states
We propose a discrete transition-based reweighting analysis method (dTRAM)
for analyzing configuration-space-discretized simulation trajectories produced
at different thermodynamic states (temperatures, Hamiltonians, etc.) dTRAM
provides maximum-likelihood estimates of stationary quantities (probabilities,
free energies, expectation values) at any thermodynamic state. In contrast to
the weighted histogram analysis method (WHAM), dTRAM does not require data to
be sampled from global equilibrium, and can thus produce superior estimates for
enhanced sampling data such as parallel/simulated tempering, replica exchange,
umbrella sampling, or metadynamics. In addition, dTRAM provides optimal
estimates of Markov state models (MSMs) from the discretized state-space
trajectories at all thermodynamic states. Under suitable conditions, these MSMs
can be used to calculate kinetic quantities (e.g. rates, timescales). In the
limit of a single thermodynamic state, dTRAM estimates a maximum likelihood
reversible MSM, while in the limit of uncorrelated sampling data, dTRAM is
identical to WHAM. dTRAM is thus a generalization to both estimators
A Fresh Variational-Analysis Look at the Positive Semidefinite Matrices World
International audienceEngineering sciences and applications of mathematics show unambiguously that positive semidefiniteness of matrices is the most important generalization of non-negative real num- bers. This notion of non-negativity for matrices has been well-studied in the literature; it has been the subject of review papers and entire chapters of books. This paper reviews some of the nice, useful properties of positive (semi)definite matrices, and insists in particular on (i) characterizations of positive (semi)definiteness and (ii) the geometrical properties of the set of positive semidefinite matrices. Some properties that turn out to be less well-known have here a special treatment. The use of these properties in optimization, as well as various references to applications, are spread all the way through. The "raison d'être" of this paper is essentially pedagogical; it adopts the viewpoint of variational analysis, shedding new light on the topic. Important, fruitful, and subtle, the positive semidefinite world is a good place to start with this domain of applied mathematics
Microbiological Implications of Periurban Agriculture and Water Reuse in Mexico City
BACKGROUND: Recycled treated or untreated wastewater represents an important health challenge in developing countries due to potential water related microbiological exposure. Our aim was to assess water quality and health implications in a Mexico City periurban agricultural area. METHODOLOGY/PRINCIPAL FINDINGS: A longitudinal study in the Xochimilco wetland area was conducted, and 42 sites were randomly selected from 211, including irrigation water canals and effluents of treatment plants. Sample collection took place during rainy and dry seasons (2000-2001). Microbiological parameters (total coliforms, fecal coliforms, streptococci/enterococci, and bacteria other than Vibrio grown on TCBS), Helicobacter pylori, and physicochemical parameters including trihalomethanes (THM) were determined. Fecal coliforms and fecal streptococci are appropriate indicators of human or animal fecal contamination. Fecal coliform counts surpass Mexican and World Health Organization irrigation water guidelines. Identified microorganisms associated with various pathologies in humans and domestic animals comprise Escherichia coli, Klebsiella spp., Salmonella spp., Enterobacter spp., Enterococcus spp., and Pseudomonas spp; H. pylori was also present in the water. An environmental characteristic of the canal system showed high Total Organic Carbon content and relatively low dissolved oxygen concentration; residual chlorine as a disinfection control is not efficient, but THMs do not represent a problem. During the rainy season, temperature and conductivity were higher; in contrast, pH, dissolved oxygen, ammonia, and residual chlorine were lower. This is related with the continuous load of feces from human and animal sources, and to the aquatic systems, which vary seasonally and exhibit evidence of lower water quality in effluents from treatment plants. CONCLUSIONS/SIGNIFICANCE: There is a need for improvement of wastewater treatment systems, as well as more efficient monitoring, regulation, and enforcement procedures for wastewater disposal into bodies of water
Social welfare and profit maximization from revealed preferences
Consider the seller's problem of finding optimal prices for her
(divisible) goods when faced with a set of consumers, given that she can
only observe their purchased bundles at posted prices, i.e., revealed
preferences. We study both social welfare and profit maximization with revealed
preferences. Although social welfare maximization is a seemingly non-convex
optimization problem in prices, we show that (i) it can be reduced to a dual
convex optimization problem in prices, and (ii) the revealed preferences can be
interpreted as supergradients of the concave conjugate of valuation, with which
subgradients of the dual function can be computed. We thereby obtain a simple
subgradient-based algorithm for strongly concave valuations and convex cost,
with query complexity , where is the additive
difference between the social welfare induced by our algorithm and the optimum
social welfare. We also study social welfare maximization under the online
setting, specifically the random permutation model, where consumers arrive
one-by-one in a random order. For the case where consumer valuations can be
arbitrary continuous functions, we propose a price posting mechanism that
achieves an expected social welfare up to an additive factor of
from the maximum social welfare. Finally, for profit maximization (which may be
non-convex in simple cases), we give nearly matching upper and lower bounds on
the query complexity for separable valuations and cost (i.e., each good can be
treated independently)
Multifrequency Photo-polarimetric WEBT Observation Campaign on the Blazar S5 0716+714: Source Microvariability and Search for Characteristic Timescales
Here we report on the results of the WEBT photo-polarimetric campaign
targeting the blazar S5~0716+71, organized in March 2014 to monitor the source
simultaneously in BVRI and near IR filters. The campaign resulted in an
unprecedented dataset spanning \,h of nearly continuous, multi-band
observations, including two sets of densely sampled polarimetric data mainly in
R filter. During the campaign, the source displayed pronounced variability with
peak-to-peak variations of about and "bluer-when-brighter" spectral
evolution, consisting of a day-timescale modulation with superimposed hourlong
microflares characterized by \,mag flux changes. We performed an
in-depth search for quasi-periodicities in the source light curve; hints for
the presence of oscillations on timescales of \,h and \,h do
not represent highly significant departures from a pure red-noise power
spectrum. We observed that, at a certain configuration of the optical
polarization angle relative to the positional angle of the innermost radio jet
in the source, changes in the polarization degree led the total flux
variability by about 2\,h; meanwhile, when the relative configuration of the
polarization and jet angles altered, no such lag could be noted. The
microflaring events, when analyzed as separate pulse emission components, were
found to be characterized by a very high polarization degree () and
polarization angles which differed substantially from the polarization angle of
the underlying background component, or from the radio jet positional angle. We
discuss the results in the general context of blazar emission and energy
dissipation models.Comment: 16 pages, 17 Figures; ApJ accepte
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