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Biomedical subjects

S A Levin

Publications and source records attributed to S A Levin.

At least 37 records · Page 2Linked to original sources
PubMed1987

Dynamical behavior of epidemiological models with nonlinear incidence rates.

Epidemiological models with nonlinear incidence rates lambda IpSq show a much wider range of dynamical behaviors than do those with bilinear incidence rates lambda IS. These behaviors are determined mainly by p and lambda, and secondarily by q. For such models, there may exist multiple attractive basins in phase space; thus whether or not the disease will eventually die out may depend not only upon the parameters, but also upon the initial conditions. In some cases, periodic solutions may appear by Hopf bifurcation at critical parameter values.

Biometry↗
PubMed1986

Influence of nonlinear incidence rates upon the behavior of SIRS epidemiological models.

When the traditional assumption that the incidence rate is proportional to the product of the numbers of infectives and susceptibles is dropped, the SIRS model can exhibit qualitatively different dynamical behaviors, including Hopf bifurcations, saddle-node bifurcations, and homoclinic loop bifurcations. These may be important epidemiologically in that they demonstrate the possibility of infection outbreak and collapse, or autonomous periodic coexistence of disease and host. The possible mechanisms leading to nonlinear incidence rates are discussed. Finally, a modified general criterion for supercritical or subcritical Hopf bifurcation of 2-dimensional systems is presented.

Humans↗
PubMed1985

Retirement centers: opportunities for the astute hospital investor.

As the population continues to age, several different forms of living arrangements will be necessary to accommodate various levels of need. Retirement communities fill the gap between living alone and living in a long-term care facility. This article reviews the market potential, past industry performance, industry structure changes, and factors for success in developing a hospital sponsored retirement community.

Aged↗
PubMed1982

Models of the influence of predation on aspect diversity in prey populations.

By considering a model incorporating behavioral changes by predators and prey evolution, we demonstrate how under certain conditions apostatic predation influences the maintenance of diversity within prey populations, and among different prey species. Further, we estimate the intrinsic "diversity" wavelength of the system when apostasy combines with other factors to lead to nonuniform distributions of prey along aspect gradients.

Animals↗
PubMed1977

Distinctions between the two-state and sequential models for cooperative ligand binding.

The two-state and sequential models for positive cooperativity in ligand binding can produce significantly different theoretical binding curves when presented in a Scatchard plot. The conditions that produce the greatest differences have been examined. The theoretical differences have been used to select the two-state model as the best model for describing the binding of acetylcholine to acetylcholine receptors that have been solubilized by Triton X-100 and sodium cholate.

Acetylcholine↗
PubMed1974

Disturbance, patch formation, and community structure.

A model is developed to relate community structure to level of environmental disturbance in systems in which the effects of disturbance are localized in space and time. In general these disturbances create a pattern of spatio-temporal heterogeneity by renewing a limiting resource, thereby permitting utilization by species that are not dominant competitors. The proposed model predicts the frequency distribution of these renewed areas, with regard to size and age (colonization stage). The model thus allows one to relate overall system pattern to the local biology within these areas, to compare various areas with different levels of disturbance, and to predict the effects of new disturbance.

Ecology↗
PubMedDate not supplied

The dynamics of group formation.

A general continuous model is presented for animal group size distribution. Attention is restricted to a fixed size population divided into groups of various dynamic sizes, but the approach extends easily to populations of variable size. The basic idea is to relate group size distribution to two functions, the (density-dependent) rates of fusion and fission. These functions can be estimated from data and can ultimately be related to the behavior of individuals and the dynamics of groups. For various functional forms, the stationary distributions of group sizes are sought. In several prototype cases, the stationary distribution has a peak value, the "most frequent group size," which emerges endogenously from the dynamics. The authors determine when such a peak emerges and more generally show the existence and uniqueness of the stationary distribution. Stability of stationary solutions is discussed. Progress is shown, but a general treatment remains refractory.

Animals↗
PubMedDate not supplied

Dynamics of influenza A drift: the linear three-strain model.

We analyze an epidemiological model consisting of a linear chain of three cocirculating influenza A strains that provide hosts exposed to a given strain with partial immune cross-protection against other strains. In the extreme case where infection with the middle strain prevents further infections from the other two strains, we reduce the model to a six-dimensional kernel capable of showing self-sustaining oscillations at relatively high levels of cross-protection. Dimensional reduction has been accomplished by a transformation of variables that preserves the eigenvalue responsible for the transition from damped oscillations to limit cycle solutions.

Antigenic Variation↗
PubMedDate not supplied

Operations improvement. Opportunities for group practices. Part II.

An Operations Improvement system developed by the New York City consulting firm APM, Inc., applies rational analytical processes to streamline daily operational tasks through a team-approach that is democratic and cost-effective. OI was explained in Part I in the March/April 1989 issue of the Group Practice Journal, Vol. 38 No. 2. The second part printed here provides specific group practice examples of application of the system and the results.

Group Practice↗
PubMedDate not supplied

[Propagation of electrical potential changes in filamentous cyanobacteria].

Possible energy transmission along conjugating membranes was investigated. It was found by extracellular electrodes that under local illumination of blue-green algae Phormidium uncinatum the electrical current propagated along trichomes. The form and sign of the responses show that e. m. f. appears in the illumination region, which generates on the cellular membrane the potential difference with a negative charge inside the cell. The initiated transmembrane potential difference propagates along the filament of Ph, uncinatum like along a cable. The hypothesis concerning a cable structure of Ph, uncinatum was supported in the experiments with intersection of algae, and with intracellular injection of fluorescein showing the existence of direct diffusion exchange through cellular interfaces.

Cell Membrane↗