Subclass Of Models Research Articles

Molecular motors and nonuniform ratchets.

Dimeric kinesin presumably moves in a "hand-over-hand" fashion via alternating steps of its two heads, which can cooperate in various ways. This motion is discussed in the framework of nonuniform ratchet models in which the molecular motor is described by M internal states and undergoes transitions at K spatial locations within the period of the molecular force potentials. Two subclasses of models with (M, K)=(3, 2) and (M, K)=(2, 2) are studied which correspond to weakly and strongly cooperative heads, respectively. Both subclasses lead to the same universal relationship between the motor velocity and the unbinding rate constant of the motor heads which is reminiscent of, but distinct from, Michaelis-Menten kinetics.

Kalman Filtering of Generalized Vasicek Term Structure Models

We present a subclass of Langetieg's (1980).linear Gaussian models of the term structure. The bond price is derived in terms of a finite set of state variables with correlated innovations. The subclass contains a reformulation of the double-decay model of Beaglehole and Tenney (1991), enabling us to clarify interpretation of their parameters. We apply Kalman filtering to a state space formulation of the model, allowing measurement errors in the data. One-, two-, and three-factor models are estimated on U.S. data from 1987–1996 and the results indicate the subclass of models can fit the U.S. term structure.

Towards mass deformed N = 4 SO( n) and Sp( k) theories from brane configurations

We study the introduction of orientifold 6-planes in the type IIA brane configurations known as elliptic models. The N = 4 SO( n) and Sp( k) theories softly broken to N = 2 through a mass for the adjoint hypermultiplet can be realized in this framework in the presence of two orientifold planes with opposite RR charge. A large class of β = 0 models is solved for vanishing sum of hypermultiplet masses by embedding the type IIA configuration into M-theory. We also find a geometric interpretation of Montonen-Olive duality based on the properties of the curves. We make a proposal for the introduction of non-vanishing sum of hypermultiplet masses in a sub-class of models. In the presence of two negatively charged orientifold planes and four D6-branes other interesting β = 0 theories are constructed, e.g. Sp( k) with four flavours and a massive antisymmetric hypermultiplet. We comment on the difficulties in obtaining the curves within our framework due to the arbitrary positions of the D6-branes.

Condensates and singularities in string theory

We derive a class of solutions to the string sigma-model equations for the closed bosonic string. The tachyon field is taken to form a constant condensate and the beta-function equations at one-loop level are solved for the evolution of the metric and the dilaton. The solutions represent critical string theories in arbitrary dimensions. The spectrum of ths subclass of models with a linearly rising asymtotic dilaton is found using the Freigin-Fuks method. Certain approximate solutions arising field theory are used to illustrate the results explicitly. An argument based on conformal invariance leads to the conjecture that stringly corrections to at least some singular spacetimes in general relativity result in non-singular metrics. We use the singularities of the big-bang/ crunch type appearing in our models to examine this conjecture.

Recognition and characterization of hierarchical interstellar structure. II - Structure tree statistics

A new method of image analysis is described, in which images partitioned into 'clouds' are represented by simplified skeleton images, called structure trees, that preserve the spatial relations of the component clouds while disregarding information concerning their sizes and shapes. The method can be used to discriminate between images of projected hierarchical (multiply nested) and random three-dimensional simulated collections of clouds constructed on the basis of observed interstellar properties, and even intermediate systems formed by combining random and hierarchical simulations. For a given structure type, the method can distinguish between different subclasses of models with different parameters and reliably estimate their hierarchical parameters: average number of children per parent, scale reduction factor per level of hierarchy, density contrast, and number of resolved levels. An application to a column density image of the Taurus complex constructed from IRAS data is given. Moderately strong evidence for a hierarchical structural component is found, and parameters of the hierarchy, as well as the average volume filling factor and mass efficiency of fragmentation per level of hierarchy, are estimated. The existence of nested structure contradicts models in which large molecular clouds are supposed to fragment, in a single stage, into roughly stellar-mass cores.

QUANTIFIED MODAL LOGIC WITH NEIGHBORHOOD SEMANTICS

AbstractThe paper presents a semantics for quantified modal logic which has a weaker axiomatization than the usual Kripke semantics. In particular, the Barcan Formula (BF) and its converse are not valid with the proposed semantics. Subclasses of models which validate BF and other interesting formulas are presented. A completeness theorem is proved, and the relation between this result and completeness with respect to Kripke models is investigated.

QUASI‐LINEAR UTILITY FUNCTIONS AND THE DYNAMICS OF PURE EXCHANGE OVERLAPPING GENERATIONS MODELS

ABSTRACTThis paper considers the dynamics of pure exchange OLG models with stationary population (each agent living for two periods), tastes and endowments, homogeneous agents and well behaved preferences. It is shown that any kind of dynamics that can emerge from this class of models can be generated by a subclass of models where preferences are described by quasi‐linear utility functions.

An incomplete data approach to the analysis of covariance structures

In this paper, linear structural equation models with latent variables are considered. It is shown how many common models arise from incomplete observation of a relatively simple system. Subclasses of models with conditional independence interpretations are also discussed. Using an incomplete data point of view, the relationships between the incomplete and complete data likelihoods, assuming normality, are highlighted. For computing maximum likelihood estimates, the EM algorithm and alternatives are surveyed. For the alternative algorithms, simplified expressions for computing function values and derivatives are given. Likelihood ratio tests based on complete and incomplete data are related, and an example on using their relationship to improve the fit of a model is given.

Assessing venture capital investments with noncompensatory behavioral decision models

This article examines the prospect of a decision support system for the assessment of venture capital investments. It approaches the task of developing decision models from the perspective of the venture capitalist. Thus venture capitalists have played an active role in defining the predictor variables and in providing data on their own investments. Venture capitalists (and recent research) underscore the importance of the entrepreneur in new ventures and the predictor variables selected reflect this bias. The literature is now replete with studies demonstrating the superiority of multiattribute actuarial models over human judgment for diverse decision environments, ranging from medical science to finance. This class of behavioral decision models consists of the simultaneous consideration of several attributes relevant to a decision. Actuarial models of this kind can be applied to attribute and decision data based on the decision maker's judgments (i.e., expected outcomes) before the events transpire—termed judgment-based models. They can also be applied to data on past events or actual outcomes where known—termed environment-based models. Again, it needs to be emphasized that actuarial models, whether based on the decision maker's judgments or on the environment, usually outperform the decision maker. These general principles are applied here. Among actuarial models a particular subclass of models is called noncompensatory. In these models, a low value for one attribute cannot be compensated by a high value for another attribute. As an example. in selecting a place-kicker in football, athletic skills other than kicking ability are irrelevant. In this study, two types of noncompensatory actuarial models, conjunctive and disjunctive, are used to model both venture capitalists' judgments and the environment. Interviews with venture capitalists reveal that some place strong emphasis on one or two attributes in judging a potential investment. This type of decision-making behavior indicates a disjunctive decision process. At the same time, there can be a tendency to ensure that a prospective investment is satisfactory at a minimum the entrepreneur's tenacity/courage is a redundant variable that is sublimated in desire for success and enthusiasm/capacity for work.

Theorems on the bounds of solutions of semi-state models

Theorems are proved that give necessary and sufficient conditions for solutions of semi-state models to be appropriately bounded on finite and infinite time intervals. For a non-linear subclass of models considered, as well as for general time-varying linear semi-state models, necessary and sufficient conditions reduce to the test of semi-definiteness or definiteness of a time-dependent matrix.

Computer Aided Determination of Model Structure for Thrombus Kinetics

An experimental goat preparation is being used to study the growth and dissolution of thrombus on different biomaterials. The incorporation of 111Indium labeled platelets into the thrombus enables the time course of tne thrombus’ growth and dissolution to be observed externally with a scintillation camera. The underlying mechanisms for the thrombus time evolution are not sufficiently well understood to allow derivation of a mathematical model from knowledge of thrombus growth biochemistry. A general thrombus-embolus model was hypothesized and a specific structure determined empirically by using a stepwise regression procedure on a class of submodels. With this approach, we have obtained a specific model structure which is identical to a single enzyme reaction equation. This model may suggest experimental methods to obtain more specific information about the underlying biochemical mechanisms.