Collective enhancement and suppression in Bose-Einstein condensates

The coherent and collective nature of Bose-Einstein condensate can enhance or suppress physical processes. Bosonic stimulation enhances scattering in already occupied states which leads to atom amplification, and the suppression of dissipation leads to superfluidity. In this paper, we review several experiments where suppression and enhancement have been observed and discuss the common roots of and differences between these phenomena.Comment: ICAP proceedings; 12 figure

Revisiting the Learnability of Apple Tasting

In online binary classification under \textit{apple tasting} feedback, the learner only observes the true label if it predicts "1". First studied by \cite{helmbold2000apple}, we revisit this classical partial-feedback setting and study online learnability from a combinatorial perspective. We show that the Littlestone dimension continues to prove a tight quantitative characterization of apple tasting in the agnostic setting, closing an open question posed by \cite{helmbold2000apple}. In addition, we give a new combinatorial parameter, called the Effective width, that tightly quantifies the minimax expected mistakes in the realizable setting. As a corollary, we use the Effective width to establish a \textit{trichotomy} of the minimax expected number of mistakes in the realizable setting. In particular, we show that in the realizable setting, the expected number of mistakes for any learner under apple tasting feedback can only be $\Theta(1), \Theta(\sqrt{T})$, or $\Theta(T)$.Comment: 18 page

Multiclass Online Learnability under Bandit Feedback

We study online multiclass classification under bandit feedback. We extend the results of Daniely and Helbertal [2013] by showing that the finiteness of the Bandit Littlestone dimension is necessary and sufficient for bandit online multiclass learnability even when the label space is unbounded. Moreover, we show that, unlike the full-information setting, sequential uniform convergence is necessary but not sufficient for bandit online learnability. Our result complements the recent work by Hanneke, Moran, Raman, Subedi, and Tewari [2023] who show that the Littlestone dimension characterizes online multiclass learnability in the full-information setting even when the label space is unbounded.Comment: 11 page

What Explains Superior Retail Performance?

We analyze the performance of retail firms for the period 1978-97 using public financial data. Our performance measures are long-term stock returns and whether the firm filed for bankruptcy in the period of study. We assume that over a long time period of at least five years, stock returns are a reasonable measure of the overall success of a firm. We have found a very wide disparity in performance between firms. On the one hand, retailers like Wal-Mart, the Gap and Circuit City have had phenomenal success (nineteen year compounded stock returns of 31.2%, 29.5%, and 34.5%, respectively), while on the other, 17% of the public retail firms filed for bankruptcy. We investigate how the following levers managed by the CEO of a retail firm affect performance: return on assets, sales growth, inventory turns, gross margin, financial leverage, and selling, general, and administrative expenses. The nature of the analysis is contemporaneous, providing insights into managerial actions that correlate with success as measured by stock returns, but not a prediction of future stock returns. We find that (1) return on assets, sales growth, standard deviation of return on assets and financial leverage explain more than 50% of the variation in stock returns for periods of ten years or more; (2) retailers in different segmentsâ apparel, department stores, grocery and convenience stores, drugs and pharmaceuticals, jewelry, consumer electronics, home furnishings, toys, and variety storesâ achieve similar return on assets and return on equity by following very different strategies with respect to their gross margins and inventory turns; (3) even within the same segment, high gross margin correlates with low inventory turns, and with high selling, general, and administrative expenses; (4) risk of bankruptcy is related to the mismatch between how fast a firm attempts to grow versus what growth rate it realizes. We also test for a negative correlation between sales growth and return on assets, which is widely believed to be true but is not borne out by our data.Operations Management Working Papers Serie

Linking Finance and Operations in Retailing

Operations Management Working Papers Serie

Signaling to Partially Informed Investors in the Newsvendor Model

We analyze a signaling game between the manager of a firm and an investor in the firm. The manager has private information about the firm\u27s demand and cares about the short-term stock price assigned by the investor. Previous research has shown that under continuous decision choices and the Intuitive Criterion refinement, the least-cost separating equilibrium will result, in which a low-quality firm chooses its optimal capacity and a high-quality firm over-invests in order to signal its quality to investors. We build on this research by showing the existence of pooling outcomes in which low-quality firms over-invest and high-quality firms under-invest so as to provide identical signals to investors. The pooling equilibrium is practically appealing because it yields a Pareto improvement compared to the least-cost separating equilibrium. Distinguishing features of our analysis are that: (i) we allow the capacity decision to have either discrete or continuous support, and (ii) we allow beliefs to be refined based on either the Undefeated refinement or the Intuitive Criterion refinement. We find that the newsvendor model parameters impact the likelihood of a pooling outcome, and this impact changes in both sign and magnitude depending on which refinement is used

Estimating Demand Uncertainty Using Judgmental Forecasts

Measuring demand uncertainty is a key activity in supply chain planning. Of various methods of estimating the standard deviation of demand, one that has been employed successfully in the recent literature uses dispersion among expertsâ forecasts. However, there has been limited empirical validation of this methodology. In this paper we provide a general methodology for estimating the standard deviation of a random variable using dispersion among expertsâ forecasts. We test this methodology using three datasets, demand data at item level, sales data at firm level for retailers, and sales data at firm level for manufacturers. We show that the standard deviation of a random variable (demand and sales for our datasets) is positively correlated with dispersion among expertsâ forecasts. Further we use longitudinal datasets with sales forecasts made 3-9 months before earnings report date for retailers and manufacturers to show that the effects of dispersion and scale on standard deviation of forecast error are consistent over time.Operations Management Working Papers Serie

What Explains Superior Retail Performance?

We analyze the performance of retail firms for the period 1978-97 using public financial data. Our performance measures are long-term stock returns and whether the firm filed for bankruptcy in the period of study. We assume that over a long time period of at least five years, stock returns are a reasonable measure of the overall success of a firm. We have found a very wide disparity in performance between firms. On the one hand, retailers like Wal-Mart, the Gap and Circuit City have had phenomenal success (nineteen year compounded stock returns of 31.2%, 29.5%, and 34.5%, respectively), while on the other, 17% of the public retail firms filed for bankruptcy. We investigate how the following levers managed by the CEO of a retail firm affect performance: return on assets, sales growth, inventory turns, gross margin, financial leverage, and selling, general, and administrative expenses. The nature of the analysis is contemporaneous, providing insights into managerial actions that correlate with success as measured by stock returns, but not a prediction of future stock returns. We find that (1) return on assets, sales growth, standard deviation of return on assets and financial leverage explain more than 50% of the variation in stock returns for periods of ten years or more; (2) retailers in different segmentsâ apparel, department stores, grocery and convenience stores, drugs and pharmaceuticals, jewelry, consumer electronics, home furnishings, toys, and variety storesâ achieve similar return on assets and return on equity by following very different strategies with respect to their gross margins and inventory turns; (3) even within the same segment, high gross margin correlates with low inventory turns, and with high selling, general, and administrative expenses; (4) risk of bankruptcy is related to the mismatch between how fast a firm attempts to grow versus what growth rate it realizes. We also test for a negative correlation between sales growth and return on assets, which is widely believed to be true but is not borne out by our data.Operations Management Working Papers Serie

Basic Atomic Physics

Contains reports on five research projects.National Science Foundation Grant PHY 96-024740National Science Foundation Grant PHY 92-21489U.S. Navy - Office of Naval Research Contract N00014-96-1-0484Joint Services Electronics Program Grant DAAHO4-95-1-0038National Science Foundation Grant PHY95-14795U.S. Army Research Office Contract DAAHO4-94-G-0170U.S. Army Research Office Contract DAAG55-97-1-0236U.S. Army Research Office Contract DAAH04-95-1-0533U.S. Navy - Office of Naval Research Contract N00014-96-1-0432National Science Foundation Contract PHY92-22768David and Lucile Packard Foundation Grant 96-5158National Science Foundation Grant PHY 95-01984U.S. Army Research OfficeU.S. Navy - Office of Naval Research Contract N00014-96-1-0485AASERT N00014-94-1-080