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At least 631 records · Page 35Linked to original sources

PubMed2005

Modeling and controlling a robotic convoy using guidance laws strategies.

This paper deals with the problem of modeling and controlling a robotic convoy. Guidance laws techniques are used to provide a mathematical formulation of the problem. The guidance laws used for this purpose are the velocity pursuit, the deviated pursuit, and the proportional navigation. The velocity pursuit equations model the robot's path under various sensors based control laws. A systematic study of the tracking problem based on this technique is undertaken. These guidance laws are applied to derive decentralized control laws for the angular and linear velocities. For the angular velocity, the control law is directly derived from the guidance laws after considering the relative kinematics equations between successive robots. The second control law maintains the distance between successive robots constant by controlling the linear velocity. This control law is derived by considering the kinematics equations between successive robots under the considered guidance law. Properties of the method are discussed and proven. Simulation results confirm the validity of our approach, as well as the validity of the properties of the method. Index Terms-Guidance laws, relative kinematics equations, robotic convoy, tracking.

Algorithms↗

PubMed1993

Bayesian inference in multipoint gene mapping.

The problem of ordering and mapping genes on the basis of recombinant data and radiation hybrid data is formulated as a problem of Bayesian inference for an unknown permutation. The challenging computational problems posed by this approach are shown to be resolvable using Markov chain Monte Carlo methods.

Algorithms↗

PubMed2000

Exposure measurement error in time-series studies of air pollution: concepts and consequences.

Misclassification of exposure is a well-recognized inherent limitation of epidemiologic studies of disease and the environment. For many agents of interest, exposures take place over time and in multiple locations; accurately estimating the relevant exposures for an individual participant in epidemiologic studies is often daunting, particularly within the limits set by feasibility, participant burden, and cost. Researchers have taken steps to deal with the consequences of measurement error by limiting the degree of error through a study's design, estimating the degree of error using a nested validation study, and by adjusting for measurement error in statistical analyses. In this paper, we address measurement error in observational studies of air pollution and health. Because measurement error may have substantial implications for interpreting epidemiologic studies on air pollution, particularly the time-series analyses, we developed a systematic conceptual formulation of the problem of measurement error in epidemiologic studies of air pollution and then considered the consequences within this formulation. When possible, we used available relevant data to make simple estimates of measurement error effects. This paper provides an overview of measurement errors in linear regression, distinguishing two extremes of a continuum-Berkson from classical type errors, and the univariate from the multivariate predictor case. We then propose one conceptual framework for the evaluation of measurement errors in the log-linear regression used for time-series studies of particulate air pollution and mortality and identify three main components of error. We present new simple analyses of data on exposures of particulate matter < 10 microm in aerodynamic diameter from the Particle Total Exposure Assessment Methodology Study. Finally, we summarize open questions regarding measurement error and suggest the kind of additional data necessary to address them.

Air Pollution↗

PubMed1998

Using a tree diagram to interpret a mixed DNA profile.

A recent case is described where the evidence of bloodstaining on a knife suggested that it was a mixture from the two victims. Interpretation of the evidence in this problem necessitated the formulation of several sets of multiple hypotheses which were analyzed by means of a tree diagram. The problem was then greatly simplified to one of comparing the two alternative hypotheses of most interest. It was found that results were robust to variation in the expert's judgment regarding the possibility that a mixture of blood was present on the knife.

Blood Stains↗

PubMed1999

Replicator equations, maximal cliques, and graph isomorphism.

We present a new energy-minimization framework for the graph isomorphism problem that is based on an equivalent maximum clique formulation. The approach is centered around a fundamental result proved by Motzkin and Straus in the mid-1960s, and recently expanded in various ways, which allows us to formulate the maximum clique problem in terms of a standard quadratic program. The attractive feature of this formulation is that a clear one-to-one correspondence exists between the solutions of the quadratic program and those in the original, combinatorial problem. To solve the program we use the so-called replicator equations--a class of straightforward continuous- and discrete-time dynamical systems developed in various branches of theoretical biology. We show how, despite their inherent inability to escape from local solutions, they nevertheless provide experimental results that are competitive with those obtained using more elaborate mean-field annealing heuristics.

Mathematics↗

PubMed1991

History and evolution of methods for solving the inverse problem.

This article serves as an introduction to the other articles in this issue devoted to the problem of the localization of neural generators. Elements of the theory of electric volume conduction are briefly introduced, as far as these apply to the interpretation of observed scalp potentials. First, some basic methods for display of the different aspects of the spatiotemporal information are described. Next, the most prominent source and volume conductor models that have been postulated for the involved forward problem are summarized. The problems of source identification and source localization, known as the inverse problem, are then formulated in terms of a parameter estimation procedure. The importance of introducing a priori information in the inverse problem, aimed at stabilizing (regularizing) the obtained solution, is emphasized. Methods for imposing such constraints are briefly outlined.

Biophysics↗

PubMed1993

[The genetic language: grammar, semantics, evolution].

The genetic language is a collection of rules and regularities of genetic information coding for genetic texts. It is defined by alphabet, grammar, collection of punctuation marks and regulatory sites, semantics. There is a review of these general attributes of genetic language, including also the problems of synonymy and evolution. The main directions of theoretical investigations of genetic language and neighbouring questions are formulated: (1) cryptographic problems, (2) analysis of genetic texts, (3) theoretical-linguistic problems, (4) evolutionary linguistic questions. The problem of genetic language becomes one of the key ones of molecular genetics, molecular biology and gene engineering.

Codon↗

PubMed2006

Searching RNA motifs and their intermolecular contacts with constraint networks.

MOTIVATION: Searching RNA gene occurrences in genomic sequences is a task whose importance has been renewed by the recent discovery of numerous functional RNA, often interacting with other ligands. Even if several programs exist for RNA motif search, none exists that can represent and solve the problem of searching for occurrences of RNA motifs in interaction with other molecules. RESULTS: We present a constraint network formulation of this problem. RNA are represented as structured motifs that can occur on more than one sequence and which are related together by possible hybridization. The implemented tool MilPat is used to search for several sRNA families in genomic sequences. Results show that MilPat allows to efficiently search for interacting motifs in large genomic sequences and offers a simple and extensible framework to solve such problems. New and known sRNA are identified as H/ACA candidates in Methanocaldococcus jannaschii. AVAILABILITY: http://carlit.toulouse.inra.fr/MilPaT/MilPat.pl.

Algorithms↗

PubMed2002

Imaging perfectly conducting objects as support of induced currents: Kirchhoff approximation and frequency diversity.

The problem of determining the shape of perfectly conducting objects from knowledge of the scattered electric field is considered. The formulation of the problem accommodates the nature of the distribution of the induced surface current density. Thus, as the unknown representing the object's contour, a single layer distribution is chosen so that the contour of the scatterer is described by its support. The nonlinear unknown-data mapping is then linearized by means of the Kirchhoff approximation, and the problem is recast as the inversion of a linear operator acting on a distribution space. An extension of the singular value decomposition approach to solve the linearized problem is provided and numerical results are presented.

Journal Article↗

PubMed2002

Robust weighted averaging.

Signal averaging is often used to extract a useful signal embedded in noise. This method is especially useful for biomedical signals, where the spectra of the signal and noise significantly overlap. In this case, traditional filtering techniques introduce unacceptable signal distortion. In averaging methods, constancy of the noise power is usually assumed, but in reality noise features a variable power. In this case, it is more appropriate to use a weighted averaging. The main problem in this method is the estimation of the noise power in order to obtain the weight values. Additionally, biomedical signals often contain outliers. This requires robust averaging methods. This paper shows that signal averaging can be formulated as a problem of minimization of a criterion function. Based on this formulation new weighted averaging methods are introduced, including weighted averaging based on criterion function minimization (WACFM) and robust epsilon-insensitive WACFM. Performances of these new methods are experimentally compared with the traditional averaging and other weighted averaging methods using electrocardiographic signal with the muscle noise, impulsive noise, and time-misalignment of cycles. Finally, an application to the late potentials extraction is shown.

Algorithms↗

PubMed2006

A binary linear programming formulation of the graph edit distance.

A binary linear programming formulation of the graph edit distance for unweighted, undirected graphs with vertex attributes is derived and applied to a graph recognition problem. A general formulation for editing graphs is used to derive a graph edit distance that is proven to be a metric, provided the cost function for individual edit operations is a metric. Then, a binary linear program is developed for computing this graph edit distance, and polynomial time methods for determining upper and lower bounds on the solution of the binary program are derived by applying solution methods for standard linear programming and the assignment problem. A recognition problem of comparing a sample input graph to a database of known prototype graphs in the context of a chemical information system is presented as an application of the new method. The costs associated with various edit operations are chosen by using a minimum normalized variance criterion applied to pairwise distances between nearest neighbors in the database of prototypes. The new metric is shown to perform quite well in comparison to existing metrics when applied to a database of chemical graphs.

Algorithms↗

PubMed1985

Compartmental analysis: theoretical aspects and application.

The basic equations governing the kinetics of metabolic compartmental systems are presented. The most difficult problem in compartmental analysis is the so-called inverse problem which is the estimation of model parameters from experimental data. Two examples are considered how to tackle this problem. The first example concerns the distribution of isotopic carbon atoms in the citric acid cycle. Starting from a matrix formulation of the problem the major steps are outlined towards an analytical solution for this rather complex system. An application of the theoretical approach to the citric acid cycle of rabbit reticulocytes is given. The second problem deals with the kinetic characterization of biochemical reactions proceeding at the same time in different physiological spaces with different rates. Such a situation is, for example, the infection of red blood cells by malaria parasites. Both the host cell and the parasite possess the complete glycolytic pathway. A theoretical approach is proposed to estimate the flux rates through this pathway separately for each of the two spaces when only the total rates and metabolite concentrations are available as experimental data.

Animals↗

PubMed1997

Physical assessment. A nutritional perspective.

Nutrition focused history and physical examination is essential in order to provide an accurate nutritional plan of care. The nutrition history provides subjective information from the patient and/or significant other in regard to nutrition related problems. Once completed, the nutrition focused physical follows corroborating the history findings and adding new findings that may assist in formulating the nutrition problem list. The goals of the nutrition focused physical examination include determining the status of nutrition processes including ingestion, digestion, absorption and metabolism, and identifying signs and symptoms of nutritional depletion or excess.

Humans↗

PubMed2005

The formulation of the control of an expression pattern in a gene network by propositional calculus.

In this study we model a gene network as a continuous-time switching network. In this model, each gene has a binary state which indicates if the gene expresses or not. We propose a method to control a sequence of expression patterns in the gene network model by adding another continuous-time switching network. By using propositional calculus, we will show that the control problem can be formulated as a mixed-integer linear programming problem with linear constraints.

Animals↗

PubMed2003

Rethinking the focusing effect in decision-making.

According to Legrenzi et al. [Cognition 49 (1993) 37], in making a choice people consider only the alternatives explicitly represented in their mental model of the decision situation. Their idea has found empirical support in the "focusing effect": Individuals focus on the alternatives explicitly stated in the problem context, and do not take into account other possibilities. In their original study, Legrenzi and colleagues considered only one factor to account for the explicit representation of an alternative--i.e., its explicit verbal formulation in the decision problem. Recent theories of relevance and information gain can help articulate their original idea, suggesting that individuals explicitly represent relevant alternatives, whether or not they are explicitly formulated in the decision problem. In three experiments we first replicated Legrenzi et al.'s original experiment, and then showed that the explicit verbal mention of an alternative is neither sufficient nor necessary to focus on it. The results suggest that individuals are able to consider relevant alternatives, even when they are not made explicit in the verbal formulation of a decision problem.

Adult↗

PubMed1978

An approach to inverse calculation of epicardial potentials from body surface maps.

The inverse problem of evaluating epicardial potentials from a knowledge of heart and torso geometry as well as body surface potentials is here formulated as a problem in control theory. As is well known, such an inverse problem is ill-posed and a regularization technique has been devised to overrun this difficulty. The resulting regularized problem is well-posed and requires the minimization of a cost function including, besides the square distance of any predicted surface potential distribution from the experimental one, a regularization term involving the second derivatives of the identified epicardial potentials. The results here presented were obtained on a model problem for a plane geometry. Surface potentials generated by multipoles and perturbated with a noise level reflecting both instrumentation and electrode placement uncertainties were fitted by the proposed method and 'epicardial potentials' were determined with a maximum sum square relative error of 15%. The results suggest that by introducing suited regularity constraints, the a priori difficulties inherent to the problem of computing epicardial potentials from torso potentials, can be overcome.

Action Potentials↗

PubMed2026

Haplotype-aware long-read error correction.

Error correction of long reads is an important initial step in genome assembly workflows. For organisms with ploidy greater than one, it is important to preserve haplotype-specific variation during read correction. This challenge has driven the development of several haplotype-aware correction methods. However, existing methods are based on either ad-hoc heuristics or deep learning approaches. In this paper, we introduce a rigorous formulation for this problem. Our approach builds on the minimum error correction framework used in reference-based haplotype phasing. We prove that the proposed formulation for error correction of reads in de novo context, i.e., without using a reference genome, is NP-hard. To make our exact algorithm scale to large datasets, we introduce practical heuristics. Experiments using PacBio HiFi sequencing datasets from human and plant genomes show that our approach achieves accuracy comparable to state-of-the-art methods. Implementation: https://github.com/at-cg/HALE .

Clustering↗

PubMed2004

The improved dissolution and prevention of ampoule breakage attained by the introduction of pretreatment into the production process of the lyophilized formulation of recombinant human Interleukin-11 (rhIL-11).

Lyophilized protein formulations sometimes pose problems such as the formation of a cloudy solution upon reconstitution. Ampoule or vial breakage can also occur during the production processes of lyophilized pharmaceutical products. Various efforts have been made to overcome those difficult problems. In this study, we introduce a particular temperature program into the production process of a recombinant human Interleukin-11 (rhIL-11) lyophilized formulation containing sodium phosphates (Na2HPO4/NaH2PO4, pH 7.0) and glycine in an attempt to improve its dissolution properties and to prevent ampoule breakage from occurring. The formulation was pretreated by nucleating ice and maintaining the solution overnight at a temperature of -6 degrees C. The solution was then completely frozen at a lower temperature. This pretreatment proved successful in not only producing a lyophilized cake which readily disintegrated and dissolved in the reconstitution media, but also prevented ampoule breakage from occurring during the production processes. In contrast, a lyophilized cake produced without the pretreatment created a cloudy solution particularly when reconstituted using water for injection contaminated with aluminum (Al3+), although the solution became transparent within 20-30 min. The pretreatment induced the crystallization of sodium dibasic phosphate (Na2HPO4) in the freeze-concentrate whereas direct freezing without the pretreatment did not crystallize the salt. Thermal analyses (DSC and TMA) showed that amorphous sodium dibasic phosphate in the freeze-concentrate became crystallized upon heating, accompanied by an increase in volume, which probably caused the ampoule breakage that occurred without the pretreatment. Although power X-ray diffraction (PXRD) experiments suggested that, with or without the pretreatment, glycine assumed the beta-form and sodium phosphate stayed amorphous in the final products, an electrostatic interaction between dibasic phosphate anions and rhIL-11, a highly cationic protein, would only exist in the lyophilized cake produced without the pretreatment. This interaction is highly likely because aluminum facilitates the formation of a cloudy solution upon reconstitution possibly by using the divalent anions which effectively reduce electrostatic repulsions between aluminum and the protein to form an aggregate structure that is not readily soluble. The pretreatment would circumvent the interaction by crystallizing the sodium salt before freezing creating a relatively soluble lyophilized cake that is much less sensitive to aluminum.

Chemistry, Pharmaceutical↗