New Approximation Algorithms for the Steiner Tree Problems

The Steiner tree problem asks for the shortest tree connecting a given set of terminal points in a metric space. We design new approximation algorithms for the Steiner tree problems using a novel technique of choosing Steiner points in dependence on the possible deviation from the optimal solutions. We achieve the best up to now approximation ratios of 1.644 in arbitrary metric and 1.267 in rectilinear plane, respectively.     

Polynomial Time Approximation Scheme for the Rectilinear Steiner Arborescence Problem

Given a set N of n terminals in the first quadrant of the Euclidean plane E ², find a minimum length directed tree rooted at the origin o, connecting to all terminals in N, and consisting of only horizontal and vertical arcs oriented from left to right or from bottom to top. This problem is called rectilinear Steiner arborescence problem, which has been proved to be NP-complete recently (Shi and Su, 11th ACM-SIAM Symposium on Discrete Algorithms (SODA), January 2000, to appear). In this paper, we present a polynomial time approximation scheme for this problem.     

Capacity inverse minimum cost flow problem

Given a directed graph G=(N,A) with arc capacities u _ij and a minimum cost flow problem defined on G, the capacity inverse minimum cost flow problem is to find a new capacity vector [^(u)]\hat{u} for the arc set A such that a given feasible flow [^(x)]\hat{x} is optimal with respect to the modified capacities. Among all capacity vectors [^(u)]\hat{u} satisfying this condition, we would like to find one with minimum ||[^(u)]-u||\|\hat{u}-u\| value. We consider two distance measures for ||[^(u)]-u||\|\hat{u}-u\| , rectilinear (L ₁) and Chebyshev (L _∞) distances. By reduction from the feedback arc set problem we show that the capacity inverse minimum cost flow problem is NP\mathcal{NP} -hard in the rectilinear case. On the other hand, it is polynomially solvable by a greedy algorithm for the Chebyshev norm. In the latter case we propose a heuristic for the bicriteria problem, where we minimize among all optimal solutions the number of affected arcs. We also present computational results for this heuristic.     

Minimum covering with travel cost

Given a polygon and a visibility range, the Myopic Watchman Problem with Discrete Vision (MWPDV) asks for a closed path P and a set of scan points $\mathcal{S}$ , such that (i) every point of the polygon is within visibility range of a scan point; and (ii) path length plus weighted sum of scan number along the tour is minimized. Alternatively, the bicriteria problem (ii??) aims at minimizing both scan number and tour length. We consider both lawn mowing (in which tour and scan points may leave?P) and milling (in which tour, scan points and visibility must stay within P) variants for the MWPDV; even for simple special cases, these problems are NP-hard. We show that this problem is NP-hard, even for the special cases of rectilinear polygons and L _?? scan range 1, and negligible small travel cost or negligible travel cost. For rectilinear MWPDV milling in grid polygons we present a 2.5-approximation with unit scan range; this holds for the bicriteria version, thus for any linear combination of travel cost and scan cost. For grid polygons and circular unit scan range, we describe a bicriteria 4-approximation. These results serve as stepping stones for the general case of circular scans with scan radius r and arbitrary polygons of feature size a, for which we extend the underlying ideas to a $\pi(\frac{r}{a}+\frac{r+1}{2})$ bicriteria approximation algorithm. Finally, we describe approximation schemes for MWPDV lawn mowing and milling of grid polygons, for fixed ratio between scan cost and travel cost.     

Min-energy broadcast in mobile ad hoc networks with restricted motion

This paper concerns about energy-efficient broadcasts in mobile ad hoc networks, yet in a model where each station moves on the plane with uniform rectilinear motion. Such restriction is imposed to discern which issues arise from the introduction of movement in the wireless ad hoc networks. Given a transmission range assignment for a set of n stations S, we provide an polynomial O(n ²)-time algorithm to decide whether a broadcast operation from a source station could be performed or not. Additionally, we study the problem of computing a transmission range assignment for S that minimizes the energy required in a broadcast operation. An O(n ³log?n)-time algorithm for this problem is presented, under the assumption that all stations have equally sized transmission ranges. However, we prove that the general version of such problem is NP-hard and not approximable within a (1?o(1))ln?n factor (unless NP?DTIME(n ^O(log?log?n))). We then propose a polynomial time approximation algorithm for a restricted version of that problem.     

Chronic stress and chronic disease in life and work: Conceptual and methodological issues

This paper argues that the lack of strong prospective evidence linking occupational and social stress to chronic disease stems from the failure of research designs to attend sufficiently to the aetiological chronicity of such diseases. Studies of both supposedly acute stress (life events) and chronic stress in life or work must increasingly be designed to distinguish between stress which is sustained or chronic over a period of yean or even decades, and hence capable of causing a serious chronic disease, and brief or transient stress, which may produce transient or brief psychological or physiological disturbances but cannot generate major chronic disease. Prospective studies are needed which collect measures of both stress and health or disease at multiple points over an extended period of time. Measures of stress should focus more on affect (for example, feelings of pressure and tension) than on cognition (for example feelings of satisfaction). Limited existing evidence is consistent with these ideals.     

The ethical implications of short-termism: Leadership failure in the executive suite

The author argues that managing to maximize short-term performance in ordinary circumstances is a failure not only of sound management practice but of moral principle. Management for the short term is largely a manifestation of self-interest which is unethical when it harms the long-term competitiveness and viability of the firm and, thus, the interests of all stakeholders but transient speculators and management itself.     

From Bottom of the Barrel to Cream of the Crop: Sequential Screening With Positive Selection

In a number of interesting environments, dynamic screening involves positive selection: in contrast with Coasian dynamics, only the most motivated remain over time. The paper provides conditions under which the principal's commitment optimum is time consistent and uses this result to derive testable predictions under permanent or transient shocks. It also identifies environments in which time consistency does not hold despite positive selection, and yet simple equilibrium characterizations can be obtained.     

MARKOV PROCESS AND CREDIT COLLECTION POLICY*

This paper deals with the evaluation of credit collection policy alternatives, given the credit granting function. Credit collection policy determines the intensity of collection efforts and the timing for recognition of bad debts. The Markov Process is utilized to estimate dynamic cash flow behavior for available policy alternatives. For this purpose, both transient and steady-state components of cash-flow behavior are considered. Operational guidelines are outlined with the help of a single example. The strength of the proposed model stems from its a) consistency with the conventional capital budgeting process, b) modest input data requirements, and c) expediency in obtaining results.     

基于VECM的汽柴油价格不对称性分析

原油作为成品油的主要原材料,原油价格的变化会引起成品油价格的相应变化,从成品油价格关于原油价格变化的反应可以了解成品油定价的合理性.本文利用一个不对称的向量误差修正模型(Vector Error CorrectionModel,VECM),检验了我国汽柴油关于原油成本变化不对称性问题,检验结果表明,我国汽柴油对原油成本上涨的反应快,但持续的时间短;对原油成本下降的反应慢,但持续的时间长.研究结果对我国石油定价机制的改革和企业实施油价风险管理有参考价值.     

A SIMULATION OF INTERACTIVE MAINTENANCE DECISIONS

In this day of automation, effective maintenance decisions are a legitimate concern. This paper broadens the traditional concept of maintenance to include any policies which tend to reduce the frequency or severity of failures. A simulation model is used to demonstrate the inter-relatedness of multiple maintenance policies for a truck depot. It is demonstrated that the proper selection of a maintenance policy set should consider the variability of the system outputs due to the daily dynamics, as well as the transient behavior of the system. It is also shown that the choice of an appropriate maintenance policy should recognize the distribution of total annual costs and should not base the analysis solely upon average total annual costs.     

Inventory Control when the Lead‐time Changes

A single‐echelon inventory system with continuous review and Poisson demand is considered. There are standard linear holding and backorder costs but no ordering or set‐up costs. We study a change in the lead‐time, which is rather typical in connection with application of a Just‐In‐Time philosophy. Our main focus is a lead‐time decrease but we also consider a lead‐time increase. Due to the lead‐time change, the optimal steady state solution will also, in general, change. We consider the transient problem of minimizing the costs when bringing the system from its original steady state to the new steady state.     

CEO political liberalism,stakeholders, and firms’ support for LGBT employees

We argue that CEO political liberalism, or the extent to which CEOs are more liberal rather than conservative, affects firms’ support for LGBT employees. Specifically, compared to conservative CEOs, liberal CEOs will be more likely to support LGBT employees by implementing LGBT-friendly policies. We furthermore posit that the presence of certain internal and external stakeholders—including boards, top management teams, and employees, as well as transient institutional investors and LGBT advocacy organizations—alter the proposed effect. Results based on a sample of Fortune 500 firms support our arguments. Our findings contribute to the literatures on leadership, stakeholders, and workplace diversity.     

A self-stabilizing 3-approximation for the maximum leaf spanning tree problem in arbitrary networks

The maximum leaf spanning tree (MLST) is a good candidate for constructing a virtual backbone in self-organized multihop wireless networks, but is practically intractable (NP-complete). Self-stabilization is a general technique that permits to recover from catastrophic transient failures in self-organized networks without human intervention. We propose a fully distributed self-stabilizing approximation algorithm for the MLST problem in arbitrary topology networks. Our algorithm is the first self-stabilizing protocol that is specifically designed to approximate an MLST. It builds a solution whose number of leaves is at least 1/3 of the maximum possible in arbitrary graphs. The time complexity of our algorithm is O(n ²) rounds.     

Dynamic Preference for Flexibility

We consider a decision maker who faces dynamic decision situations that involve intertemporal trade‐offs, as in consumption–savings problems, and who experiences taste shocks that are transient contingent on the state of the world. We axiomatize a recursive representation of choice over state contingent infinite horizon consumption problems, where uncertainty about consumption utilities depends on the observable state and the state follows a subjective Markov process. The parameters of the representation are the subjective process that governs the evolution of beliefs over consumption utilities and the discount factor; they are uniquely identified from behavior. We characterize a natural notion of greater preference for flexibility in terms of a dilation of beliefs. An important special case of our representation is a recursive version of the Anscombe–Aumann model with parameters that include a subjective Markov process over states and state‐dependent utilities, all of which are uniquely identified.     

Long‐Term Risk: An Operator Approach

We create an analytical structure that reveals the long‐run risk‐return relationship for nonlinear continuous‐time Markov environments. We do so by studying an eigenvalue problem associated with a positive eigenfunction for a conveniently chosen family of valuation operators. The members of this family are indexed by the elapsed time between payoff and valuation dates, and they are necessarily related via a mathematical structure called a semigroup. We represent the semigroup using a positive process with three components: an exponential term constructed from the eigenvalue, a martingale, and a transient eigenfunction term. The eigenvalue encodes the risk adjustment, the martingale alters the probability measure to capture long‐run approximation, and the eigenfunction gives the long‐run dependence on the Markov state. We discuss sufficient conditions for the existence and uniqueness of the relevant eigenvalue and eigenfunction. By showing how changes in the stochastic growth components of cash flows induce changes in the corresponding eigenvalues and eigenfunctions, we reveal a long‐run risk‐return trade‐off.     

Happiness at Work

Happiness in the form of pleasant moods and emotions, well‐being, and positive attitudes has been attracting increasing attention throughout psychology research. The interest in happiness has also extended to workplace experiences. This paper reviews what is known about the definition, causes and consequences of happiness at work, drawing also on insights from the expanding positive psychology literature on happiness in general. Many discrete organizational behavior constructs arguably belong to a larger family of happiness‐related constructs, and share some common causes and consequences. Happiness at work includes, but is far more than, job satisfaction. A comprehensive measure of individual‐level happiness might include work engagement, job satisfaction, and affective organizational commitment. Aspects of happiness have been (and should be) conceptualized and measured at multiple levels, including transient experiences, stable person‐level attitudes, and collective attitudes, and with respect to multiple foci, such as discrete events, the job, and the organization. At all levels, there is evidence that happiness has important consequences for both individuals and organizations. Past research has tended to underestimate the importance of happiness at work.     

Organizational Learning under Organizational Impermanence: Collaborative Ties in Film Project Firms

In the last two decades a lot of research has been devoted to unveiling the processes through which organizations learn and store knowledge. This research is typically concerned with organizations lastingly engaged in the provision of goods or services. Permanency is usually presumed in order for the encoding of inferences from history to take place. But what if organizational permanency cannot be assumed ex-ante? Project firms represent an interesting case in point. A project firm is a transient form of organization that ceases to exist as soon as its single target is achieved, as such it does not exhibit stable structures nor does it exhibit ostensible history-based paths upon which to build its choices and nurture its organizational knowledge. This apparent paradox can be resolved, in part, by extending the view from the isolated project to the relational context in which project firms operate. Using longitudinal data from the U.S. feature film industry, we show that the process of organizational formation and dissolution that characterizes this context is underpinned by patterns of enduring collaborations among interdependent industry participants. We build on these findings to speculate on processes of learning and remembering that interpenetrate project firms’ boundaries, by being embedded within a texture of stable interpersonal ties.     

Postnatal Effects of Prenatal Insult

Exogenous agents may perturb development during the embryonic period and adversely affect the formation of organs. However, adverse effects on development are not limited to the embryonic period nor are the manifestations restricted solely to outright gross structural malformation, but may instead be expressed as a decrement or abberration of postnatal function. Susceptibility to altered development may extend well into the postnatal period. Studies of functional parameters in several organ systems have demonstrated the broad-based susceptibility, subtlety of expression and potential of long-lasting effects of altered development assessed by physiologic assays. Adverse effects on functional development, whether in the CNS, reproductive, gastrointestinal, genitourinary, respiratory, or immune systems, etc., merit continuing investigation. From the viewpoint of risk estimation and hazard detection, evaluations of postnatal functional parameters may be relevant for several reasons. First, such parameters may serve as low-dose triggers. Second, they may be useful as a focal point for epidemiological studies. Finally, a more thorough understanding of the degree and magnitude of such postnatal functional deficits is needed since an adverse maternal effect may be transient, considered acceptable, or unperceived, but the effect on the conceptus may be permanent and severe. The immune and respiratory systems are discussed as two examples of how subtle and protean adverse effects on functional development may be.     

A Multilayered Box Model for Calculating Preliminary Remediation Goals in Soil Screening

In the process of screening a soil against a certain contaminant, we define the health-risk-based preliminary remediation goal (PRG) as the contaminant concentration above which some remedial action may be required. Thus, PRG is the first standard (or guidance) for judging a site. An overestimated PRG (a too-large value) may cause us to miss some contaminated sites that can threaten human health and the environment. An underestimated PRG (a too-small value), on the other hand, may lead to unnecessary cleanup and waste tremendous resources. The PRGs for soils are often calculated on the assumption that the contaminant concentration in soil does not change with time. However, that concentration usually decreases with time as a result of different chemical and transport mechanisms. The static assumption thus exaggerates the long-term exposure dose and results in a too-small PRG. We present a box model that considers all important transport processes and obeys the law of mass conservation. We can use the model as a tool to estimate the transient contaminant concentrations in air, soil, and ground water. Using these concentrations in conjunction with appropriate health-risk parameters, we may estimate the PRGs for different contaminants. As an example, we calculated the tritium PRG for residential soils. The result is quite different from, but within the range of, the two versions of the corresponding PRG previously recommended by the U.S. EPA.