Optimal Job Design and Career Dynamics in the Presence of Uncertainty

The paper studies a learning model in which information about a worker's ability can be acquired symmetrically by the worker and a firm in any period by observing the worker's performance on a given task. Productivity at different tasks is assumed to be differentially sensitive to a worker's intrinsic talent: potentially more profitable tasks entail the risk of greater output destruction if the worker assigned to them is not of the ability required. We characterize the (essentially unique) optimal retention, task assignment and promotion policy for the class of sequential equilibria of this game, by showing that the equilibria of interest are strategically equivalent to the solution of an experimentation problem (a discounted multi-armed bandit with independent and dependent arms). These equilibria are all ex ante efficient but involve ex post inefficient task allocation and separation. While the ex post inefficiency of separations persists even as the time horizon becomes arbitrarily large, in the limit task assignment is efficient. When ability consists of multiple skills, low performing promoted workers are fired rather than demoted, if outcomes at lower level tasks, compared to those at higher level tasks, provide a sufficiently accurate measure of ability. We then examine the strategic effects of the dynamics of learning on a worker's career profile. We prove, in particular, that price competition among firms causes ex ante inefficient turnover and task assignment, independently of the degree of transferability of human capital. In a class of equilibria of interest it generates a wage dynamics consistent with properties observed in the dataLearning, Job Assignment, Experimentation, Correlated Multi-armed Bandit

Brushes of semiflexible polymers in equilibrium and under flow in super-hydrophobic regime

We performed molecular dynamics simulations to study equilibrium and flow properties of a liquid in a nano-channel with confining surfaces coated with a layer of grafted semiflexible polymers. The coverage spans a wide range of grafting densities from essentially isolated chains to dense brushes. The end-grafted polymers were described by a bead spring model with an harmonic potential to include the bond stiffness of the chains. We varied the rigidity of the chains, from fully flexible polymers to rigid rods, in which the configurational entropy of the chains is negligible. The brush-liquid interaction was tuned to obtain a super-hydrophobic channel, in which the liquid did not penetrate the polymer brush, giving rise to a Cassie-Baxter state. Equilibrium properties such us brush height and bending energy were measured, varying the grafting density and the stiffness of the polymers. We studied also the characteristics of the brush-liquid interface and the morphology of the polymers chains supporting the liquid for different bending rigidities. Non-equilibrium simulations were performed, moving the walls of the channel in opposite directions at constant speed, obtaining a Couette velocity profile in the bulk liquid. The molecular degrees of freedom of the polymers were studied as a function of the Weissenberg number. Also, the violation of the no-slip boundary condition and the slip properties were analyzed as a function of the shear rate, grafting density and bending stiffness. At high grafting densities, a finite slip length independent of the shear rate or bending constant was found, while at low grafting densities a very interesting non-monotonic behaviour on the bending constant is observed

Structure, thermodynamic properties, and phase diagrams of few colloids confined in a spherical pore

We study a system of few colloids confined in a small spherical cavity by event driven molecular dynamics simulations in the canonical ensemble. The colloidal particles interact through a short range square-well potential, which takes into account the basic elements of attraction and excluded-volume repulsion of the interaction among colloids. We analyze the structural and thermodynamic properties of this few-body confined system in the framework of the theory of inhomogeneous fluids. Pair correlation functions and density profiles across the cavity are used to determine the structure of the system and the spatial characteristics of its inhomogeneities. Pressure on the walls, internal energy and surface quantities such as surface tension and adsorption are also analyzed for the whole range of densities, temperatures and number of particles considered. We have characterized the structure of systems from 2 to 6 confined particles as function of density and temperature, identifying the distinctive qualitative behaviors all over the thermodynamic plane $T-\rho$ in a few-particle equivalence to phase diagrams of macroscopic systems. Applying the extended law of corresponding states the square well interaction is mapped to the Asakura-Oosawa model for colloid-polymer mixtures. We link explicitly the temperature in the confined square-well fluid to the equivalent packing fraction of polymers in the Asakura-Oosawa model. Using this approach we study the confined system of few colloids in a colloid-polymer mixture.Comment: 17 pages, 2 tables, and 17 figure

Free energy calculations of elemental sulphur crystals via molecular dynamics simulations

Free energy calculations of two crystalline phases of the molecular compound S8 were performed via molecular dynamics simulations of these crystals. The elemental sulphur S8 molecule model used in our MD calculations consists of a semi-flexible closed chain, with fixed bond lengths and intra-molecular potentials for its bending and torsional angles. The intermolecular potential is of the atom-atom Lennard-Jones type. Two free energy calculation methods were implemented: the accurate thermodynamic integration method proposed by Frenkel and Ladd and an estimation that takes into account the contribution of the zero point energy and the entropy of the crystalline vibrational modes to the free energy of the crystal. The last estimation has the enormous advantage of being easily obtained from a single MD simulation. Here we compare both free energy calculation methods and analyze the reliability of the fast estimation via the vibrational density of states obtained from constrained MD simulations. New results on alpha- and alpha'- S8 crystals are discussedComment: 18 pages, 2 figures, submitted to J. Chem. Phy

Trophonella (Gastropoda: Muricidae), a New Genus from Antarctic Waters, with the Description of a New Species

The new genus Trophonella is described from the outer shelf and upper continental slope of Antarcticaand islands within the Antarctic Convergence. Four previously known species that had been attributed to the genusTrophon (Trophon scotianus Powell, 1951; T. echinolamellatus Powell, 1951; T. enderbyensis Powell, 1958; and T.eversoni Houart, 1997) are included in Trophonella, as is one new species (Trophonella rugosolamellata) describedherein. Trophonella resembles Trophon in gross shell morphology: the members of both genera have large, globoseshells, paucispiral protoconchs, prominent axial lamellae, and short siphonal canals. Trophonella differs fromTrophon in having shells with evenly rounded whorls that lack a well-defined shoulder; rachidian teeth withdistinctive, broadly triangular central cusps, but that lack the marginal cusps of Trophon; characteristic sphericalaccessory salivary glands; and a circumpapillar fold on the penis that is absent in Trophon. Relationships of thegenera Trophon and Trophonella, as well as of the subfamily Trophoninae are reexamined by supplementing the datamatrix of Kool (1993b, Table 3) with data for additional taxa. Results support the segregation of Trophonella fromTrophon at the generic level. Based on the relationships of the type species of their respective nominotypical genera,Trophoninae is either the sister taxon of a narrowly circumscribed Ocenebrinae, or both are part of a larger clade. Abetter resolved phylogeny containing a much broader sampling of the more than 50 genus-level taxa that have beenattributed to these two subfamilies will be required in order to delineate more precisely the membership of the cladeand to identify its diagnostic synapomorphies.Fil: Harasewych, M. G.. National Museum of Natural History; Estados UnidosFil: Pastorino, Roberto Santiago Guido. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia”; Argentin

Mechanical performance of auxetic polyurethane foam for antivibration glove applications

In this study the static and dynamic characteristics of conventional open cell polyurethane (PU), of auxetic (negative Poisson’s ratio) and of iso-density foams were analysed. The specimens were produced from conventional gray open-cells polyurethane foam with 30-35 pores/inch and 0.0027 g/cm3 density, by means of process which has been previously defined by the authors. Poisson’s ratio measurements were performed under quasi-static conditions using an MTS 858 servohydraulic test machine and a video image acquisition system. For the auxetic foams the results suggested similar behaviour to that previously reported in the literature, with significant increases in stiffness during compressive loading, and a significant dependence of the Poisson’s ratio on the applied strain. Transmissibility tests, performed in accordance with the ISO 13753 procedure for antivibration glove materials, suggested a strong dependence of the transmissibility on the foam manufacturing parameters. Within the frequency range from 10 to 31.5 Hz the transmissibility was found to be greater than 1, while it was less than 1 at all frequencies greater than 31.5 Hz. The transmissibility results were similar to the mean values for 80 resilient materials tested by Koton et. al., but were higher than the five best materials (not all polymeric) identified by the same researchers. In this study it has been suggested that the resilient behaviour of glove isolation materials should also be evaluated in terms of the indentation characteristics. A simple, linear elastic, Finite Element simulation was therefore performed, and the indentation results suggested that auxetic foams offer a significant decrease in compressive stresses with respect to conventional PU foams

Learning-by-employing: the value of commitment

We analyze a dynamic principal–agent model where an infinitely-lived principal faces asequence of finitely-lived agents who differ in their ability to produce output. The ability of anagent is initially unknown to both him and the principal. An agent’s effort affects the informationon ability that is conveyed by performance. We characterize the equilibrium contracts andshow that they display short–term commitment to employment when the impact of effort onoutput is persistent but delayed. By providing insurance against early termination, commitmentencourages agents to exert effort, and thus improves on the principal’s ability to identify theirtalent. We argue that this helps explain the use of probationary appointments in environmentsin which there exists uncertainty about individual ability.Keywords: dynamic principal–agent model, learning, commitment.

Test of a simple and flexible molecule model for alpha-, beta- and gamma-S8 crystals

S8 is the most stable compound of elemental sulfur in solid and liquid phases, at ambient pressure and below 400K. Three crystalline phases of S8 have been clearly identified in this range of thermodynamic parameters, although no calculation of its phase diagram has been performed yet. alpha- and gamma-S8 are orientationally ordered crystals while beta-S8 is measured as orientationally disordered. In this paper we analyze the phase diagram of S8 crystals, as given by a simple and flexible molecule model, via a series of molecular dynamics (MD) simulations. The calculations are performed in the constant pressure- constant temperature ensemble, using an algorithm that is able to reproduce structural phase transitions.Comment: RevTex,7 pages, 5 figures,to appear in J. Chem. Phy

A Reconstruction Procedure for Microwave Nondestructive Evaluation based on a Numerically Computed Green's Function

This paper describes a new microwave diagnostic tool for nondestructive evaluation. The approach, developed in the spatial domain, is based on the numerical computation of the inhomogeneous Green’s function in order to fully exploit all the available a-priori information of the domain under test. The heavy reduction of the computational complexity of the proposed procedure (with respect to standard procedures based on the free-space Green’s function) is also achieved by means of a customized hybrid-coded genetic algorithm. In order to assess the effectiveness of the method, the results of several simulations are presented and discussed

Detection of Buried Inhomogeneous Elliptic Cylinders by a Memetic Algorithm

The application of a global optimization procedure to the detection of buried inhomogeneities is studied in the present paper. The object inhomogeneities are schematized as multilayer infinite dielectric cylinders with elliptic cross sections. An efficient recursive analytical procedure is used for the forward scattering computation. A functional is constructed in which the field is expressed in series solution of Mathieu functions. Starting by the input scattered data, the iterative minimization of the functional is performed by a new optimization method called memetic algorithm. (c) 2003 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other users, including reprinting/ republishing this material for advertising or promotional purposes, creating new collective works for resale or redistribution to servers or lists, or reuse of any copyrighted components of this work in other works