Browsing by Author "Bhootra, Ajay"

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    A Disassembly Optimization Problem
    Bhootra, Ajay (Virginia Tech, 2002-09-20)
    The rapid technological advancement in the past century resulted in a decreased life cycle of a large number of products and, consequently, increased the rate of technological obsolescence. The disposal of obsolete products has resulted in rapid landfilling and now poses a major environmental threat. The governments in many countries around the world have started imposing regulations to curb uncontrolled product disposal. The consumers, too, are now aware of adverse effects of product disposal on environment and increasingly favor environmentally benign products. In the wake of imminent stringent government regulations and the consumer awareness about ecosystem-friendly products, the manufacturers need to think about the alternatives to product disposal. One way to deal with this problem is to disassemble an obsolete product and utilize some of its components/subassemblies in the manufacturing of new products. This seems to be a promising solution because products now-a-days are made in accordance with the highest quality standards and, although an obsolete product may not be in the required functional state as a whole, it is possible that several of its components or subassemblies are still in near perfect condition. However, product disassembly is a complex task requiring human labor as well as automated processes and, consequently, a huge amount of monetary investment. This research addresses a disassembly optimization problem, which aims at minimizing the costs associated with the disassembly process (namely, the costs of breaking the joints and the sequence dependent set-up cost associated with disassembly operations), while maximizing the benefits resulting from recovery of components/subassemblies from a product. We provide a mathematical abstraction of the disassembly optimization problem in the form of integer-programming models. One of our formulations includes a new way of modeling the subtour elimination constraints (SECs), which are usually encountered in the well-known traveling salesman problems. Based on these SECs, a new valid formulation for asymmetric traveling salesman problem (ATSP) was developed. The ATSP formulation was further extended to obtain a valid formulation for the precedence constrained ATSP. A detailed experimentation was conducted to compare the performance of the proposed formulations with that of other well-known formulations discussed in the literature. Our results indicate that in comparison to other well-known formulations, the proposed formulations are quite promising in terms of the LP relaxation bounds obtained and the number of branch and bound nodes explored to reach an optimal integer solution. These new formulations along with the results of experimentation are presented in Appendix A. To solve the disassembly optimization problem, a three-phase iterative solution procedure was developed that can determine optimal or near optimal disassembly plans for complex assemblies. The first phase helps in obtaining an upper bound on our maximization problem through an application of a Lagrangian relaxation scheme. The second phase helps to further improve this bound through addition of a few valid inequalities in our models. In the third phase, we fix some of our decision variables based on the solutions obtained in the iterations of phases 1 and 2 and then implement a branch and bound scheme to obtain the final solution. We test our procedure on several randomly generated data sets and identify the factors that render a problem to be computationally difficult. Also, we establish the practical usefulness of our approach through case studies on the disassembly of a computer processor and a laser printer.
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    Two Essays on Momentum and Reversals in Stock Returns
    Bhootra, Ajay (Virginia Tech, 2008-05-01)
    This dissertation consists of two essays. In the first essay, I examine the source of momentum in stock returns. The reversal of momentum returns has been interpreted as evidence that momentum results from delayed overreaction to information. I examine momentum and reversals conditional on firms’ share issuance (net of repurchases) during the momentum holding period and show that (1) among losers, the momentum returns are statistically significant, but the reversals are non-existent, for both issuers and non-issuers; (2) among winners, momentum and reversals are restricted to issuers, but are non-existent among non-issuers. After further conditioning on firm size, I find that winner reversals are restricted to small, equity issuing firms. After excluding these small issuers from the sample, the remaining firms have strong momentum profits with no accompanying reversals. The evidence suggests that the return reversals are a manifestation of the poor performance of equity issuing firms. Further, while investor overreaction potentially contributes to the momentum among winners, a large fraction of firms do not earn any significant abnormal returns following initial price continuation, suggesting that underreaction, and not delayed overreaction to information, is the dominant source of momentum in stock returns. In the second essay, I examine alternative explanations of reversals in stock returns. George and Hwang (2007) find that long-term reversals in stock returns are driven by investors’ incentive to defer payment of taxes on locked-in capital gains rather than by overreaction to information. I show that return reversals are instead attributable to the negative relationship between firms’ composite share issuance and future stock returns documented in Daniel and Titman (2006). The ability of locked-in capital gains measures to forecast stock returns is largely subsumed by the composite share issuance measure. My results do not support the hypothesis that capital gains taxes drive long-term return reversals.